The bioconcentration and metabolism of ¹⁴C-labeled trans-isomers of tetramethrin (I) [3,4,5,6-tetrahydrophthalimidomethyl (1RS)-trans-chrysanthemate] in bluegill sunfish (Lepomis macrochirus) were examined under flow-through conditions. Ester cleavage followed by rapid liberation of a hydroxymethyl group from the alcohol moiety resulted in the formation of trans-chrysanthemic acid (VI) and 3,4,5,6-tetrahydrophthalimide (III) as primary metabolites in fish. III was either hydrolyzed to the diacid (V) or reduced at the 1,2-double bond (IV). VI and its metabolites formed through successive oxidation at the isobutenyl moiety were further conjugated with glucuronic acid or taurine. The rapid and extensive metabolism of trans-I with a short depuration half-life of 0.54–0.72 days resulted in a much lower bioconcentration factor (BCF), 180–310, than expected from its hydrophobicity (log K_ow=4.6). The BCF value of its cis-isomer was conveniently estimated to be 230–400 by taking into account the different degradation rates between both isomers evaluated through an in vitro metabolism study.