AbstractA factorial design was used to determine the effects of age (2, 4, and 8 d for flagfish; 13, 20, and 26 d for white sucker) and ration (fed or unfed) on the acute toxicity of permethrin pulse exposure. A similar design was used to evaluate the acute toxicity of glyphosate to flagfish. Relative tolerance was assessed by determining the 2‐h pulse‐exposure concentration causing 50% mortality (PE LC50) over the subsequent 96 h. Age at exposure and the presence or absence of food modified the toxicity of permethrin to both flagfish and white sucker. White sucker were consistently less tolerant of permethrin than flagfish. Fed and unfed 8‐d‐old flagfish, as well as unfed 2‐d‐olds, showed equivalent levels of permethrin tolerance, with respective 96‐h PE LC50s of 0.57, 0.54, and 0.68 mg L−1. They were significantly less tolerant than 4‐d‐old unfed juveniles (2.97 mg L−1), which were in turn significantly less tolerant than 2‐ and 4‐d‐old fed juveniles, whose respective 96‐h PE LC50s were 5.55 and 7.91 mg L−1. Unfed white suckers were less tolerant than fed white suckers at all ages tested. Unfed white suckers at 13 d (96‐h PE LC50, 0.002 mg L−1) and 20 d (0.001 mg L−1) were significantly less tolerant than unfed fish at 26 d (0.172 mg L−1) and fed fish at 13 d (0.184 mg L−1), which were more tolerant than 20‐d‐old fed fish (0.010 mg L−1). Fed fish at 26 d of age were the most tolerant (3.668 mg L−1). Glyphosate proved to be relatively nontoxic to flagfish up to 30 mg L−1. No mortality was observed during the bioassays with 2‐ and 4‐d‐old fed and starved fish. Fed 8‐d‐olds (96‐h PE LC20, 29.6 mg L−1) were significantly more tolerant of glyphosate than were unfed 8‐d‐olds (96‐h PE LC20, 2.94 mg L−1). The sharply delineated periods of permethrin sen