The effects of chronic irradiation on the life-time breeding performance of the small tropical fish, Poecilia reticulata, have been investigated at mean dose-rates of 0.17, 0.40 and 1.27 rad hour-1. The total fecundity was markedly (P less than 0.001) reduced at all dose-rates owing to a decrease in mean actual brood-size and an increase in temporary and permanent infertility. Minor progressive changes in the interbrood time with age and dose-rate were noted. The neonatal death-rate, incidence of abnormalities, and survival and sex ratio of the offspring were unaffected by irradiation. The brood-size data have been used to derive estimates of the dominant lethal mutation rate which are of the same order as those determined for mammals. Histological studies indicate that functional sterility is not necessarily dependent on the destruction of the gonads, and it is possible that radiation effects on pituitary function are responsible for much of the observed infertility.