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Over the past twelve years, we have studied heat shock proteins in two tropical species, a half dozen desert species and a number of hemiclones of viviparous fishes in the genus Poeciliopsis, Heat shock protein (Hsp) isoform patterns were determined using high resolution two-dimensional polyacrylamide gels. Two families of Hsps were studied in detail, the nucleocytoplasmic 70 kilodalton Hsp70 family and the 30 kilodalton Hsp30 family related to alpha-crystallin. The temperature dependence of Hsp accumulation was investigated using both intact fish and cultured cells. When the threshold temperatures were mapped onto thermal preference profiles, it was apparent that the Hsp70 threshold (33 degrees C) was closely linked to the most frequently selected temperatures and the Hsp30 threshold (37 degrees C) was closely linked to high temperatures that fish rarely selected, indicating that fish deploy these two molecular chaperones differently. One tropical species P. gracilis is a genetic reservoir for most of the Hsp70 isoforms of the desert species. Acquired resistance to 41 degrees C was strongly correlated with Hsp70 abundance for gracilis that contained Hsp70 isoform 3 whereas fish lacking this isoform showed similar levels of acquired thermotolerance which did not correlate with Hsp70 abundance, suggesting multiple, compensating mechanisms of acquired resistance, Isoform 3 was degraded in cultured cells from a desert species during several hours of recovery at normal temperature following heat shock whereas two other Hsp70 isoforms were stable. The implications of this property of isoform 3 are discussed.