Life tables have been prepared for a fish (Lebistes reticulatus) under various laboratory conditions. The age-mortality relationship in this species does not appear to differ materially from that in small mammals. Mortality increases regularly with age, and the change in the survival curve with improving conditions is like that found in animals of fixed life-span; the limit in Lebistes is 2000 days. The frequency of correlated deaths due to epizootic incidents likewise increases with increasing age. The relation between the curves of age/size and age/mortality in Leblstes is consistent with that which has been postulated for larger fish (plaice, sturgeon). Pathological findings in the population under study are described.