Vanadium concentrations of 2.4 to 5.6 mg/L were lethal in 7 days to rainbow trout of wet weight 1.2-6.2 g. The LC50 varied slightly over the 12 combinations of water quality, from hardness 30 to 350 units and pH 5.5 to 8.8. The 7-day LC5O may be estimated by the following equation, which explained 91% of the variation: LC50 = 14.6976 - 3.7783P + 0.1108H - 0.02137 PH + 0.2662p2 - 0.000073H2 + 0.00141p2H. where H = hardness as mg/L of CaC03, and P = pH. The response surface was slightly saddle-shaped with vanadium being somewhat more toxic in the softest water, and slightly more toxic at intermediate pH (6.6 and 7.7) than at more extreme values of pH. Two ionic species of pentavalent vanadium were the main forms present in the tests with trout, and these were of similar toxicity. No threshold of lethality was evident in an 11-day exposure. Very young fry of American flagfish showed 28-day LCSO IS from 0.9 to 1.9 mg/L of vanadium, according to size and age at the start of the test. These appeared to be thresholds of lethality. Trout, flagfish, and zebrafish all appeared to be similar in resistance to lethal effects of vanadium. In chronic exposures of flagfish, the egg-fry stage was the most sensitive one in the life cycle. Mortality of such fry was the most obvious effect. At 0.17 mg/L of vanadium, which did not cause mortality, there were marginal effects on growth of second generation fry, but no observed sublethal effects in older fish. At 0.04 mg/L there were no deleterious effects, but a definite stimulation of growth in females and of reproductive performance. The threshold for chronic toxicity was between those two concentrations, and was judged to be about 0.08 mg/L. The "safe"-to lethal ratio v was about 0.007, close to such ratios for other pollutants. There was no evidence that vanadium had any long-term cumulative toxicity. Overall among the metals, vanadium was of moderate noncumulative toxicity. With respect to oil sands operations, there should be an assessment whether aerial fallout of vanadium could create undesirable levels in slow-turnover lakes.