Toxicity tests are most frequently conducted on organisms derived from laboratory cultures that have been reared for many generations following standardized protocols. The restricted density of laboratory populations and the possibility of occasional population crashes increases the probability that random genetic drift and inbreeding may decrease genetic variation of the culture below that typically found in natural populations of the species. Likewise, isolation of organisms in different laboratories may contribute to significant genetic differentiation among culture populations... In the situation where certain genotypes are more resistant to a particular stressor, using a culture that is dominated by the resistant genotype may underestimate the toxic effect of a contaminant. Similarly, the inclusion of a greater number of sensitive genotypes in a test may overestimate toxicity. This experiment was conducted to test the null hypothesis that no difference in tolerance to zinc contamination would be noted among distinct genotypes of H. azteca obtained from a single laboratory source. Zinc was selected as the contaminant in this study because much of the focus in research conducted to examine genotypic differences in stressor susceptibility has centered on heavy metals. Although zinc is an essential trace element for organisms, it can , at high concentrations, act as an environmental contaminant, particularly in aquatic ecosystems that receive mining effluent. Hyallela azteca was chosen because the research of Duan et al. (1997) indicated genetic differentiation among laboratory cultures and this species is commonly used in ecotoxicological studies.