Genetic homogeneity among endrin-resistant and -susceptible pine vole populations and between voles that. survived or died on restricted feed intake was tested using starch-gel electrophoresis of 5 polymorphic enzyme markers. Three apple orchards annually treated with endrin for the past 15 years and 3 non-treated orchards in southwestern Virginia were sampled in Fall 1980, and in 2 of each orchard type in Spring 1981 (N=786). LD-50’s were uniformly low in all Fall orchards, increasing in only one endrin-treated orchard by Spring 1981 (from 2.4 to 64.5 mg/kg). Orchard, season, and genotype variables were jointly dependent at all 5 loci, but average heterozygosity was similar among the orchards (decreasing from 0.490 to 0.470 in sprayed orchards and from 0.548 to 0.494 in unsprayed orchards). The one incidence of increased endrin tolerance entailed no unique genetic component related unequivocally to endrin treatment. Genetic heterogeneity was suggested among 36 pine voles subjected to 70% ad libitum feed intake for 24 days. Although approximately equally composed of males and females, only one-fourth of the 33.3% that died were females. Weight losses did not differ significantly between the survivors and casualties on restriction (22% and 24% of initial weight, respectively). Mean individual heterozygosity was greater for females (0.809) than for males (0.625) over all groups, and greater in both sexes for survivors of the feed restriction (0.742) compared to those that died (0.576), but these differences were statistically nonsignificant. The data suggested a relationship between survival and heterozygosity that deserves closer study. / Master of Science