Statistically significant, positive correlations were found between individual heterozygosity at seven protein polymorphisms and size in five natural populations of tiger salamanders. When these same populations were sampled later in the same year, the positive associations between heterozygosity and size were no longer evident. A pair mating was conducted in the laboratory, and offspring were distributed randomly to four replicate populations maintained in the laboratory. In two to these four replicates, a significant, positive association between protein heterozygosity and size was observed. The laboratory results suggest that the associations seen in natural populations are due to associations between protein heterozygosity and growth rate early in the larval period.