Comparisons were made of the mortality, growth and condition factor over a 50‐day period of eleutheroembryos, alevins, 2‐month fry, 4‐month fry and 1 + rainbow trout,Salmo gairdneri, in water of three qualities–Birmingham tap‐water, 50% and 100% settled effluent from percolating filters (Minworth Effluent Treatment Works) when dissolved oxygen levels were maintained near 100% ASV.There was no significant mortality among life stages in control tap‐water but the LT50 values of life stages in both 50% effluent and 100% effluent revealed significant differences. Inter‐treatment comparisons showed similar mortalities of eleutheroembryos, of 2‐month fry and of 1 + fish in the three treatments but LT50 values of 4‐month fry showed significant differences. Growth rates of 2‐month and 4‐month fry were suppressed in both effluents after 7 days and there was a significant deterioration in theKfactor of these life stages after 14 days. Predicted toxicities (the sum of the proportions of the 48‐h LC50 of the individual poisons to rainbow trout) were compared with the observed toxicity in 50% and 100% effluent. The relevance ofin vivotesting in comparison to predictive