Rainbow trout lactate dehydrogenase (LDH) B2' B2' , B2' B2" and B2" B2" phenotypes were tested under a variety of conditions for swimming endurance, oxygen consumption rates, and blood pH, p02, and lactate. The B2" B2" phenotype exhibited a lower swimming endurance, under a limited oxygen supply, than the B2' B2' and B2' B2" phenotypes. Blood pH at fatigue did not differ among the LOH phenotypes that swam under low oxygen conditions (2 mg/1). Blood p02 at fatigue was significantly higher for females than for males under low oxygen conditions. Oxygen consumption rates were significantly higher for males than for females at dissolved oxygen concentrations of 3 mg/1 to 8 mg/1. The depression in oxygen consumption rates at low oxygen levels was greater for males than for females. The increase in blood lactate with swimming under low oxygen concentrations and static exposure to low oxygen depended on LOH phenotype. The adaptive significance of the LDH B2' and B2" alleles was discussed. Thirteen populations of Intermountain cutthroat trout were sampled for biochemical genetic variation. Variation was observed in muscle aspartate aminotransferase for all populations which would fit a genetic model of two loci and three alleles. Unique muscle A group lactate dehydrogenase variation was observed in Snake Valley cutthroat trout which suggested the presence of several variant alleles. Allele frequencies for tetrazolium oxidase changed during three consecutive years in Bear Lake cutthroat trout that were trapped and spawned artificially. The possible causes of those changes were discussed. No variation was observed in alphaglycerophosphate dehydrogenase, isocitrate dehydrogenase, eye lactate dehydrogenase or muscle malate dehydrogenase in cutthroat trout.