This dissertation tells the story of the doctors and biologists who worked for the Medical Section of the Manhattan Engineer District of the United States Army Corps of Engineers during and after World War II. It tracks how this unlikely and close-knit network of medical doctors and fisheries biologists used x-rays to irradiate animals to produce data that anticipated the exposure of human beings and entire environments to radiation from fission. By charting the development, spread, and evolution of the x-ray animal research tradition, this dissertation reveals how a unique type of biology grew up within the US atomic program in the 1940s and 1950s. This story has largely fallen out of the canonical atomic narrative told in the US. I argue that Medical Section functionaries developed and then manipulated the x-ray animal research tradition to claim biological expertise over questions that arose at early US atomic sites. They deployed their knowledge of the biological effects of radiation to support the early atomic project, making the case that plutonium production and atomic testing could be safely accomplished. Theirs was science developed to support federal goals. This dissertation uses the work of the Medical Section’s most important installation, the Applied Fisheries Laboratory at the University of Washington, as the core of its narrative. I follow the expansion of the laboratory’s research program across the US’s nascent atomic geography. The scientists took their research toolkit to the plutonium production site at Hanford, to Hiroshima and Nagasaki after the first atomic bombs fell, and to the test sites at Bikini, Enewetak, and Rongelap Atolls in the Marshall Islands. Medical Section practitioners studied the biotic populations in all these places after exposure to radiation from fission had transformed them and their biotic populations. These scientists continued their work even after the US Congress disestablished the Manhattan Engineer District in 1946, replacing it with the Atomic Energy Commission. Charting their path as they became scientific experts at atomic sites shows how federally funded biology helped underpin the United States’ quest for nuclear hegemony during World War II and the Cold War.