A series of reintroduction programs have reestablished fisher (Martes pennanti) populations to a large part of their former range. Horizontal starch-gel electrophoresis of 20 presumptive gene loci was used from 1996 to 1998 to investigate gene dynamics in 4 remnant fisher populations (sources) and 4 reintroduced fisher populations from the northeastern and Great Lakes regions of the United States. Mean multilocus heterozygosities, mean number of alleles per locus, and percentage polymorphic loci were 0.027–0.090, 1.2–1.6, and 10.0–30.0, respectively, in the 8 populations surveyed. Significant allelic frequency differentiation was detected among the 4 source populations and among the 4 reintroduced populations, but a significant proportion of the genetic variance was partitioned only among the 4 reintroduced populations. Pairwise comparisons between each source and its associated reintroduced population indicated that only older reintroductions have attained significant differentiation of allelic frequency from their sources. Significant heterozygotic deficiencies were detected for statewide populations and regional populations within states, suggesting that breeding biology of the fisher, presumably among females, is creating levels of fine-scale genetic structure within populations.