The effects of acute beta and gamma radiation on developing Cyprinus carpio eggs were measured by the percentage of the eggs that hatched. Eggs were exposed to various doses of beta and gamma radiation from external sources thirty minutes after fertilization. LD 50 doses established by a probit analysis were 500.5 rads for beta and 601.3 rads for gamma radiation.

The radiation resistance of the common mosquito fish, Gambusia affinis affinis, from two natural populations was compared by establishing <tex-math>${\rm LD}_{50(30)}$</tex-math> for each population. One population had been exposed to chronic environmental radiation of approximately 10 rads/day for over 100 generations, while the other population was exposed only to natural background radiation. The <tex-math>${\rm LD}_{50(30)}$</tex-math> for each population were ascertained at environmental temperatures of 20, 25, or 30°C. A statistical analysis of the data based on a modified form of a logistic dose response model showed no significant difference in radiation response between the two populations. However, as the temperature increased above 20°C, the sensitivity of Gambusia to radiation increased. Differences of only 5°C changed significantly the <tex-math>${\rm LD}_{50(30)}$</tex-math> of each population.

The fecundity of Gambusia affinis affinis, the common mosquito fish, that has inhabited a lake contaminated with radioactive waste for over one hundred generations, was compared with a control population. The calculated dose rate from the bottom sediments to the Gambusia population in this contaminated environment was 10.9 rads/day. A significantly larger brood size occurred in the irradiated than in the nonirradiated population, although significantly more dead embryos and abnormalities were observed in the irradiated broods. These results suggest that an increased fecundity is a means by which a natural population of organisms having a relatively short life cycle and producing a large number of progeny can adjust rapidly to an increased mortality caused by radiation.