Exposure of rainbow trout (Salmo gairdneri) starting at the egg stage to water-borne lead in Lake Ontario water (hardness of 135 mg CaCO3/L) resulted in black tails (early symptoms of spinal deformities) at lead concentrations as low as 22 μg/L. No black tails were observed at 11 μg/L. Previous publications demonstrated that the lowest lead concentration causing black tails in trout exposed from the fingerling stage in Lake Ontario water was 120 μg/L, suggesting that fish exposed from the egg stage are 5 times as sensitive as those exposed from the fingerling stage. Compared to previously published data on fish exposed from the egg stage, blood lead of fish exposed from the egg stage was greater for a similar lead exposure. However, the relationship between blood lead and enzyme inhibition (erythrocyte δ-amino levulinic acid dehydratase) was unchanged. Therefore the increased sensitivity of trout exposed from the egg stage was probably due to enhanced lead accumulation rather than to an enhanced response to the lead taken up. The incidence of black tails at this hardness corresponded well to a prediction based on published lead toxicity at extremes of water hardness.