ABSTRACT.1. Overwintering survivorship of pupae of the mimosa webworm,Homadaula anisocentraMeyrick (Lepidoptera: Plutellidae), was examined in several urban habitats in central Iowa during the winters of 1981–82, 1982–83 and 1983–84.2. Survivorship and supercooling point temperatures were determined throughout the winters. Corroborative laboratory studies were conducted during the winter of 1982–83.3. Minimum ambient temperatures that equalled or were below the supercooling point of the insect, at any time, were lethal.4. Prolonged cold exposure below 0°C and above the supercooling point resulted in high mortality levels. To quantify this relationship, a concept of minimum‐temperature exposure was developed by tabulating the number of degrees that the daily minimum temperature was below 0°C for a given sampling period.5. Some mimosa webworm pupae were found to overwinter in highly protected sites (2.5–5.0°C warmer than the ambient air temperatures) in the urban environment, resulting in less minimum‐temperature exposure and reducing the probability of reaching the lethal supercooling p