As a part of the Marine Life Research Program of the Scripps Institution of Oceanography (a member of the California Coöperative Oceanic Fisheries Investigations) an increased effort is being made to describe and evaluate the various organic factors that are important in the biological economy of the sea. In attacking the problem, the most expedient procedure is to study in detail the various components of the plankton, for it is well known that these components in varying degrees of importance provide directly the basic food for the plankton-feeding fishes, such as the sardine, and also directly or indirectly for larger but less abundant pelagic fishes, as well as benthonic life. Among the several groups of zoöplankton organisms that are being studied are the euphausiid shrimp. These highly pelagic crustaceans, popularly known as “krill,” occur in large swarms in all oceans in both neritic and oceanic waters. They are considered second in importance only to the copepods as basic animal food in the sea but often exceed the copepods in mass and numbers, especially at greater depths. The present report deals with the group taxonomically on a wide geographic basis. The chief purpose here is to provide an essential tool with complete descriptions and illustrations to facilitate further study of the biology of the euphausiids and their relation to the pelagic community of the Pacific. Such a study is now under way, and it is believed that application of knowledge of the various species, their geographic ranges, concentrations, and reproductive areas will yield pertinent information relative to the importance of different oceanic currents and water masses in the marine ecology of our coasts. Among the marine animals that are known to feed upon the euphausiids are especially such fish as the herring and sardine, and also the whalebone whales whose diet is, despite their huge size, almost exclusively plankton. A literature too voluminous to review here bears this out for both the fish and mammals. The catch of baleen whales is remarkably closely correlated with the abundance and swarming of euphausiids in the Atlantic and Antarctic waters. Locally, the California gray whale is known to feed at least in part upon Euphausia pacifica, the most common euphausiid off our coast. Similarly, the humpback whales of our coast were found to have fed upon “shrimp.” The place of euphausiids in the diet of the California sardine and other local fishes is presently under investigation in connection with the Marine Life Research program. The euphausiids are food not only to fish and other aquatic animals but to a lesser extent also to sea birds. The euphausiids usually live at depths beyond the range of surface-feeding animals, but during swarming, vast hoards may migrate to the very surface within reach of large flocks of birds. These swarmings may be within restricted areas as in the passages between Passamaquoddy Bay and the Bay of Fundy. Here Meganyctiphanes norvegica and Thysanoëssa spp. rise in the turbulent water within reach of sea gulls and other birds. On the open coast off La Jolla, California, swarms of female Thysanoëssa spinifera that had migrated to the upper water layers were swept ashore and stranded upon the beach. In Australian waters “rafts” of Nyctiphanes australis, covering an acre in extent, may occur at the surface where they are fed upon by the muttonbird. Finally, euphausiids may hold a place directly in the diet of man himself. Japanese fishermen sometimes catch vast numbers of these crustaceans, which periodically swarm in the deep bays of Honshu Island. The niche occupied by the euphausiids in the food chain of the sea requires also a consideration of the food upon which they themselves subsist. Some progress along this line has been made by various workers. Much remains to be done. Einarsson (1945) reviews briefly pertinent literature which brings evidence to show that the diet of euphausiids consists of both diatoms and microcrustaceans, but chiefly of the phytoplankton diatoms. Floating detritus is also an item of diet, but further study is needed in determining the efficiency of the feeding mechanisms in screening out the very finest particulate organic material known to be present in the sea water. The feeding mechanism of the euphausiids consists of a basket formed by the thoracic appendages, the inner margins of which are provided with long plumose bristles or setae. Water is strained through the bristles, and the small organisms and detritus are screened out as the animal swims through the water propelled by the swimming feet (pleopods) on the abdomen. Important also are the mouth parts. The mouth parts are situated immediately in front of the thoracic appendages. From anterior to posterior, they consist of labrum, mandibles (with “teeth” and palps), labia, first maxillae, and second maxillae. There are no maxillipeds differing from the thoracic appendages. The first and second maxillae are small, but they are well provided with many rather short setae and spines which are apparently used mainly in holding or transferring food particles to the labia and mandibles. The active feeding process of euphausiids was not studied but, from consideration of the structure of the feeding mechanisms, it appears that at least in Euphausia pacifica an important nonscreening device is the pair of mandibular palps. These palps are designed to move the food particles from the basket and maxillae into reach of the labia and mandibles and doubtless to hold larger particles against the mandibles during mastication. The end segment of each palp is provided with a row of strong, curved spines along the posterior edge. The palps are rotated somewhat toward the mid-line to show the orientation of the spines. In Nematoscelis difficilis the mandibular palps are much shorter and may therefore serve a more restricted function. In the genus the second pair of thoracic legs are enormously elongated, but it is not clear with what feeding function this modification may be associated. Although much of the food material consists of detrital particles or organisms of very small size, this is only a fraction of the diet. Larger objects such as copepod larvae, and even adults, form a part of the food of Meganyctiphanes norvegica. The well-developed mandibles with chitinized gnathobase attest to this fact. They are each provided with a cutting incisor process and a molar process useful in crushing.