Under spring conditions (mean daily maximum 22 C, mean daily minimum 9 C), the temperature within duck carcasses paralleled air temperature for 3 days; on days 4 and 5 the internal temperature rose above 30 C for approximately 30 hr and maximum temperatures of 40-47 C occurred. This coincided with the period of maximum blowfly maggot activity in the carcasses. Carcasses screened from blowflies did not experience this period of high internal temperature. Under autumn conditions (mean daily maximum 13 C, mean daily minimum 1 C), the internal temperature of carcasses paralleled air temperature for approximately 2 wk. Following a warm day (23.5 C), maggots appeared in the carcasses and the internal temperature rose markedly higher than air temperature. Maggots moved into the soil on cold nights and reinhabited the carcasses during the day. The microclimate within maggot-infested carcasses appeared very suitable for growth and toxin production by Clostridium botulinum and this phenomenon may help explain the occurrence of botulism outbreaks during cool weather.