The relationship between allozyme genotype and survival of nymphs of the mayfly Isonychia bicolor (Walker) when exposed to acute mercury concentrations was tested in laboratory experiments. The probability of survival and individual times-to-death were found to be significantly different among genotypes at the glucose phosphate isomerase locus, but not at the phosphoglucomutase locus. This pattern was consistent for both summer and winter temperatures and for populations from 2 separate, unpolluted streams. Subsequent field surveys were conducted to determine if results of these acute laboratory tests could be extrapolated to the more chronic, sublethal conditions present in the South River, Virginia. Genotypes identified as sensitive and tolerant in the laboratory experiments showed no consistent relationship with environmental mercury levels in the South River. In fact, one heterozygote identified as tolerant was found to be more frequent at the reference site than at contaminated sites, and no significant between-site differences were observed in the frequencies of the most sensitive genotype. Consequently, despite fitness differences to mercury exposure among allozyme variants, we were unable to attribute any between-site differences in genetic structure in I. bicolor populations to adaptation to mercury pollution.