Continuous exposure to 28% refinery effluent caused severe sublethal damage to flagfish ( Jordanella floridae), including smaller size of first- and second-generation fish, hemorrhaging of fins in all fish, spinal curvature in the second generation, as well as delayed and less frequent spawning with fewer eggs per spawn, compared to the controls. Effluent at 0.92% caused no deleterious effects. The threshold for sublethal effect was approximately 9%, which caused only one deletrious effect of questionable overall importance, out of 15 major items assessed. The sublethal findings are considered representative for a well-treated refinery effluent; this one had average values for ammonia, pH, and oil-plus-grease that were close to or within Canadian regulations, sulphide and phenol levels that were lower than the regulation, and total residue somewhat higher than regulations. For the 30 twice-weekly samples used in this experiment, mortality of rainbow trout ( Salmo gairdneri) in full-strength effluent averaged only 5% and no single sample caused > 50% mortality. Treatment of refinery effluent to meet Canadian regulations would mean that at most refinery sites, effluent would be diluted to the no-effect levels found in this experiment.