Five groups of esterase active bands were found after electrophoretic separation of extracts of heart tissue of lobsters (Homarus americanus) on starch gels. Each group consisted of one or more bands that were distinguishable from other groups by their electrophoretic mobility, substrate specificity, and susceptibility to various inhibitors. Groups designated I–IV are carboxylesterases; group V is a cholinesterase. Individual variations were observed in groups I, II, and V. The observed polymorphisms in groups I and II were hypothesized as due to codominant allelic systems. The phenotypic frequencies of group I isozymes observed in two populations agreed well with the Hardy–Weinberg distributions expected for such systems. The frequencies of variant types in group II were too low for meaningful population genetic analysis. Phenotype frequencies were not correlated with sex, size of the carapace, or stage of the molt cycle. Developmental changes of esterase isozymes were observed during the first eight stages. The Es-V (cholinesterase) group and Es-II groups were present at hatching, but the Es-I group was not clearly observed until after the seventh molt. Phenotype ratios of offspring from single females supported the genetic hypothesis for the Es-I system.