Two decapod crustaceans, the grass shrimp,Palaemonetes pugio, and the fiddler crab,Uca pugnax, were subjected to graded doses of cobalt-60 gamma radiation. The dose required to kill 50% of the animals within 7 days, 14 days, and 30 days was calculated. The LD50/30 days ofPalaemonetes pugio was about 1,500 r and the LD50/30 days ofUca pugnax was about 8,000 r. Females ofP. pugio which were ovigerous at the time of irradiation either released their eggs unhatched, or, in the one case in which hatching occurred, all larvae died within three days. Prolongation of the intermolt period was observed inU. pugnax individuals irradiated at the lower dosages.