We determined the survival, development, and fecundity of Dermatophagoides farinae Hughes exposed to fluctuating daily regimes of hydrating and dehydrating relative humidity. Larva emerged from 84, 92, and 94% of eggs incubated at a regime of 2, 4, and 8 h at 75% RH and 22, 20, and 16 h at 0% RH, respectively. No emerging offspring completed the life cycle when exposed to the 2 and 4 h of moist air daily but 44 and 53% survived for 70 d in the larval or nymphal stages, respectivley, and these completed development to adults when subsequently held at a constant 75% RH. Given 8 h of moist air daily, 41% of emerging offspring completed the life cycle but development was 1.6 times longer compared with development at a constant 75% RH. For all daily hydrating and dehydrating regimes, a greater percentage of offspring became males than females. Overall, survival of immatures was remarkable at these daily long periods of dehydrating conditions when a short period of hydrating moisture was provided. When exposed to a daily regime of 4 h of moist air (75% RH) and 20 h of dry air (0% RH), 84% of females survived 28 d and produced approximately 1/3 of the number of eggs produced at constant 75% RH (control).