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.

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