SummaryVariation in the proportions of serum esterase genetic polymorhisms drawn from samples of skipjack tuna (Katsuwonus elamis) has been used to identify geneticaty isolated stocks ofK. pelamisin the South Pacific, based on the assumptions that variation in serum esterase gene proportion was essentially stochastic; such aruments are shown to be unacceptable. There is evidence that serum esterases may be involved in the deknsive resonses of vertebrates to parasites, and that genetic variation at a number of loci may be related to growth rate. It can be shown in a large proportion of samples that serum esterase gene proportion inK. pelamiswas correlated to measures of growth rate. This implies that the Pacific‐wide variation in esterase proportions in skipjack tuna may be due to local selection gradients involving growth rat