The Gulf of Thailand and the South China Sea were each sampled during five cruises of the Naga Expedition, 1959-1961. The following twenty species were present: Cavolina gibbosa, C. globulosa, C. inflexa, C. longirostris, C. uncinata, Clio recurva, C. cuspidata, C. pyramidata, Creseis acicula, C. chierchiae, C. virgula, C. bulgia, Cuvierina columnella, Diacria quadridentata, D. trispinosa, Hyalocylis striata, Limacina bulimoides, L. inflata, L. trochiformis, and Styliola subula. Distribution maps are presented for each species and variations in distribution are related to climatic and hydrographic parameters. Statistical treatment of the data helped to establish recurrent species groups among ten selected species and to relate the distribution of these groups to environmental configurations. Broadly speaking, there are two recurrent species groups. The first is restricted to the deeper-water areas of the South China Sea, while the second occurs in both the Gulf of Thailand (a shallow, estuarine environment) and all of the South China Sea. The members of the first group, Group A, are Clio pyramidata, Creseis virgula virgula +; C. virgula conica, Limacina bulimoides, L. inflata, and Styliola subula. This group has the narrowest environmental tolerances. It does not occur in the low salinity, high temperature, low oxygen environment of the Gulf of Thailand and the extent of its penetration of the Sunda Shelf area of the southern South China Sea varies with season. Because of their relative rarity in the samples Cavolina gibbosa, C. globulosa, C. inflexa, C. uncinata, Clio recurva, C. cuspidata, Cuvierina columnella, and Diacria trispinosa were not included in the statistical treatment, but their distribution patterns place them with Group A. The second broad group has been divided into two, based on the extent of tolerance for the most extreme conditions in the Gulf of Thailand. Group B, consisting of Diacria quadridentata, Hyalocylis striata, and Limacina trochiformis, is less tolerant of extreme Gulf conditions than the following Group, C, and tends to be concentrated in the central, deeper area of the Gulf of Thailand. H. striata was absent from the Gulf during January-May, 1960 (Cruises S-3 and S-5), and D. quadridentata was absent during April-May (S-5). The members of Group C, Creseis acicula and Cavolina longirostris, have the broadest environmental tolerances and were present and abundant even at nearshore stations in the Gulf of Thailand where salinities were below 29 o/oo;. Group C tended to be most abundant around the margins of the Gulf. The members of both Groups B and C were widely distributed in the South China Sea as well.