The intracellular build-up of thermally damaged proteins following exposure to heat stress results in the synthesis of a family of evolutionarily conserved proteins called heat shock proteins (Hsps) that act as molecular chaperones, protecting the cell against the aggregation of denatured proteins. The transcriptional regulation of heat shock genes by heat shock factor 1 (HSF1) has been extensively studied in model systems, but little research has focused on the role HSF1 plays in Hsp gene expression in eurythermal organisms from broadly fluctuating thermal environments. The threshold temperature for Hsp induction in these organisms shifts with the recent thermal history of the individual but the mechanism by which this plasticity in Hsp induction temperature is achieved is unknown. We examined the effect of thermal acclimation on the heat-activation of HSF1 in the eurythermal teleost Gillichthys mirabilis. After a 5-week acclimation period (at 13, 21 or 28 degrees C) the temperature of HSF1 activation was positively correlated with acclimation temperature. HSF1 activation peaked at 27 degrees C in fish acclimated to 13 degrees C, at 33 degrees C in the 21 degrees C group, and at 36 degrees C in the 28 degrees C group. Concentrations of both HSF1 and Hsp70 in the 28 degrees C group were significantly higher than in the colder acclimated fish. Plasticity in HSF1 activation may be important to the adjustable nature of the heat shock response in eurythermal organisms and the environmental control of Hsp gene expression.