We sought to determine the dietary folic acid requirement of young rainbow trout using growth indices supported by measurements of tissue folate concentrations. The investigation was conducted with purified diets that had, by assay, basal folic acid levels of 0.08 and 0.16 mg/kg in the first and second, respectively, of two experiments. Each experiment was started with fry (initial mean weight, 1.4 and 2.8 g/fish in Experiments 1 and 2, respectively) and was conducted at a water temperature of 15 degrees C. Experiment 1 lasted 18 wk and Experiment 2 lasted 16 wk. Recovery tests (of 8 wk duration, performed on fish fed the unsupplemented diet) and pair-feeding showed that the unsupplemented diet led to a folate-specific deficiency condition in which the main hematological abnormality was the appearance of misshapen nuclei in a small proportion (2.3%) of erythrocytes. Dietary requirements were shown not to exceed 0.3 and 0.6 mg folic acid/kg (17 and 33 micrograms/MJ digestible energy) for optimizing survival and growth indices, respectively. We conclude that the dietary folate requirement of the trout is comparable to that of other vertebrates for the purpose of achieving maximal weight gain.