The effects of elevated temperatures on largemouth bass (Micropterus s. salmoides), from Minnesota and Wisconsin (our northern stock) and from Tennessee (our southern stock), were compared at four first-year life stages. The purpose of these tests was to determine the degree of applicability of data gathered from a sample of fish from one part of its range of distribution to thermal standard setting for that same species at some other location within its normal range. Eggs and embryos, when first exposed early in development to an array of elevated temperatures, lost 50% of their maximum hatching success (TL50) at 29.5C for the northern stock and at 29.1C for the southern stock.