Temperature effects on hematological responses of adult bluegill were examined for constant vs fluctuating temperature regimes, for ambient vs high temperature, and for intake vs discharge at an operating power plant. Multivariate statistical methods were used to determine temperature effects. An increased erythrocyte fragility was associated with fluctuating temperature regimes as well as with the suboptimal temperature regime. Thrombocytosis with a concomitant leukopenia was observed for the higher fluctuating and higher acclimating temperatures. Reduced erythrocytic sedimentation rates were observed for the higher temperatures. Although hematology may vary depending on fish size, good hematological indicators of potential thermal stress were associated with red blood cell morphology, particularly cell size and/or shape. However, in situ hematological responses of bluegill at an operating power plant were apparently not affected by the associated thermal gradients.