House dust mites are found in almost all dwellings in New Zealand and are a major risk factor in the development of asthma and perennial allergic rhinitis. We studied the longevity, life stage length, and fecundity of a New Zealand strain of European house dust mite, Dermatophagoides pteronyssinus (Trouessart), at constant (23°C, 75% RH) and the fluctuating conditions typically found in dry (18–25°C, 60–38% RH) and damp (18–23°C, 70–55% RH) New Zealand dust mite microhabitats in carpets. All the adult mites placed in the “dry” conditions died within 18 d. Mites in the “damp” conditions had developmental times, oviposition, and death rates that were not significantly different from constant conditions. These mites are tolerant of fluctuating temperatures, but they are more susceptible to environments that strongly fluctuate in humidity.