Protein products of 24 loci from the genomes of Yellowstone Park elk were analyzed by electrophoresis. Heterozygosity was detected in only one system, making elk much less polymorphic than eastern whitetailed deer. Data for several other large mammals are compared with those for elk and reveal similarly low levels of isozymic variation. The data are consistent with the fine-grained niche theory but difficult to reconcile with bottlenecks and genetic drift.