AbstractThe temperature dependence of liver microsomal CYP2E1 (cytochrome P450 2E1) activity was examined in selected genotypes of the viviparous fishPoeciliopsisActivity of this enzyme, as a function of incubation temperature, was determined by measuring 6 OH‐chlorzoxazone formation from chlorzoxazone, a specific CYP2E1 substrate Chlorzoxazone 6 hydroxylase activity was examined among five species ofPoeciliopsis, as well as among nine genotypes within a species,P monachaAmongPoeciliopsisgenotypes,P monachacontained the greatest activity, 9 5 ± 1 5 U (where 1 U is defined as 1 μg 6 OH chlorzoxazone produced/min/mg protein,n= 6) with a temperature optimum (To) of 25°C The lowest activity was inP occidentalis, 0 65 ± 0 11 U, with aToof 27 to 28°C,P prolifica, P fasciata, P lucida, andP viriosahad intermediate levels of activities, 1 1 to 5 5 U, andTofrom 25 to 31°C To determine if metabolic differences exist within species, enzyme activity was examined from nine genotypes ofP monachaby comparing expression amongP monacha‐lucidahybrids These hybrids were given identical paternal genomes oflucidabut retained the original maternalmonachagenomes sampled from wild gene pools The greatest activity was found in genotype T70 3PCw, 3 6 ± 0 1 U, at aToof 29°C, and the lowest was in genotype SV73 7s, 0 40 ± 0 12 U, at aToof 27°C The other naturally occurring genotypes, M65 24, M65–26 SV73 7v, as well as the laboratory produced synthetic hybrids, Syn4 and Syn5, had intermediate activities, 0 73 ± 0 38 to 2 1 ± 0 69 U, andToof 25 to 27°C No hybrid had activity levels as high as the maternal parent,P monacha, and only one had aToas low as either parent Apparently the genes involved in xenobiotic activity vary widely among the closely related species ofPoeciliopsisbut also within species suggesting that these phenotypes can be acted upon b