Field observations indicated considerable loss of eggs of smallmouth bass and a high incidence of fungus infection in the nests in Cayuga Lake, New York, and tributaries. To investigate the possible effect of temperature changes on embryo survival, several tests were run using constant-temperature facilities at the Cornell University Insectary. In 1945, two trials were run raising the temperature of developing ova from 53° to 77° F., and no eggs were killed. During 1946, eggs developing at 65° F. were transferred at 0, 1, 2, 3 and 4 days following fertilization to 50° and 75° F. None of the eggs was adversely affected. The temperature changes took place within one-half hour. The mean incubation period was determined to vary from about 2 1/4 days at a constant temperature of 75° F. to nearly 10 days at 55° F. The observations presented lead to the conclusion that temperature changes in the ranges indicated are not directly responsible for the death of smallmouth bass embryos.