Rainbow trout (initial weight of 4 or 5 g) were acclimated at a cool, 11.0 degrees C (C), a warm, 18.0 degrees C (W), or an intermediate temperature 14.5 degrees C (I) for 1 month. There was a slight difference in hepatic microsomal content of one of six cytochrome P450 isozymes between acclimation groups. Monounsaturated fatty acids in hepatic phosphotidylethanolamine but not phosphotidylcholine increased at lower acclimation temperatures. Saturated fatty acid content decreased with temperature for both phospholipid classes. Fish were exposed to 0.08-0.12 ppm waterborne aflatoxin B1 (AFB1) for 30 min at respective acclimation temperatures or after acute temperature shifts (24 hr) and reared for 9 months at C, I, or W. With exposure concentrations which delivered equivalent target organ doses, trout acclimated, exposed, and reared at C, I, or W had liver tumor incidences of 4, 35, and 61%, respectively. The average number of tumors per liver increased from 1.25-1.34 at C to 2.46-2.66 at W. There were no temperature-dependent differences in tumor diameter. When C- and W-acclimated fish were AFB1 exposed and reared at I, tumor incidence was 12.5% for W-I-shifted fish and 26.5% for C-I-shifted fish. This was consistent with previous work which demonstrated acute downward temperature shift reduced [3H]AFB1 adduction to hepatic DNA. Tumor incidence and multiplicity data suggested manipulation of temperature permitted selective modulation of cancer initiation and promotion in rainbow trout.