The acute toxicity of methiocarb in juvenile rainbow trout (Oncorhynchus mykiss, 3.25 +/- 0.79 g) was evaluated in glass aquaria under static conditions. Nominal concentrations of methiocarb in the toxicity test ranged from 1.25 to 7.50 mg L-1. The concentrations of methiocarb that killed 50% of the rainbow trout within 24-h (24-h LC50), 48-h LC50, 72-h LC50, and 96-h LC50 were 5.43 +/- 0.19, 5.04 +/- 0.18, 4.95 +/- 0.19, and 4.82 +/- 0.21 mg L-1 (95% confidence limits), respectively. Mortality of fish increased with increasing water temperature. Increasing alkalinity from 19 mg L-1 as CaCO3 to 40, 60, or 90 mg L-1 as CaCO3 significantly decreased mortality of fish. Total hardness ranging from 50 mg L-1 as CaCO3 to 147 mg L-1 as CaCO3 did not affect mortality of fish exposed to methiocarb. Fish exposed to methiocarb had histological alterations such as lamellar edema, separation of epidermis from lamellae, and lamellar fusion. Methiocarb exposed fish had necrosis between molecular and granular layer of cerebellum where Purkinje cells present. Results indicate that alkalinity, temperature, and fish size affect methiocarb toxicity of rainbow trout. (c) 2005 Elsevier B.V. All rights reserved.