The thermoinactivation ofAmyeloiscalicivirus (ACV) was tested with samples incubated at 40 and 45°C for 2 to 8 h, 50°C for 0.25 to 6 h, and 55°C for 1 to 60 min. Larval thermal resistance was investigated at constant temperatures of 30, 32, and 34°C. Comparative potencies of viral samples after heat treatment were estimated from dose-mortality assays and from measurements of larval weight and length as an indication of the relative intensity of larval stunting caused by chronic infection. There was no significant (P>0.05) change in viral activity at 40°C, but a sharp decline was noted after 6 h at 45°C. Similarly, viral activity was almost completely lost after 1 h at 50°C. There was a lag phase during the first 10 min at 55°C, but then activity declined rapidly to reach minimal value after about 45 min incubation. Thermal resistance of larvae was strongly evident at 34°C and virus-treated larvae were not significantly different (P>0.05) from untreated controls. In some respects, the thermal responses of ACV were similar to those reported for infectious f1acherie virus of the silkworm and the pathogenic nonoccluded virus of the citrus red mite