The 96-h LC50 of vanadium to adult American flagfish (Jordanella floridae) was 11.2 mg l−1 in very hard water. Larvae showed 28-day LC50's of 1.13 and 1.88 mg l−1 of vanadium with larger larvae being more resistant. These appeared to be thresholds of lethality. During continuous exposure for 96 days, larval growth and survival were the most sensitive indicators of vanadium toxicity and were marginally reduced at 0.17 mg l−1. At 0.041 mg l−1, there were no deleterious sublethal effects but there was definite stimulation of growth in females and of reproductive performance. The threshold for chronic toxicity was judged to be about 0.08 mg l−1. The “safe”-to-lethal ratio was 0.007 and this could be used as an application factor for other species. There was no clear evidence that vanadium had any long-term cumulative toxicity.