Life-history parameters ofMononychellus progresivusDoreste andOligonychus gossypii(Zacher), 2 major mite pests of cassava in Africa, were determined in the laboratory at 3 constant relative humidities (30, 60, and 90% RH) obtained with saturated salt solutions. Experiments were carried out in airtight boxes placed in an air-conditioned room at 26 ± 1°C and a photoperiod of 12:12 (L:D) h (illuminance 3,500 lux). Low (30% HH) and high (90% RH) air humidity had a negative effect on the life-history traits of both species compared with medium air humidity (60% RH). For both species, the strongest effect was obtained at 90% RH; e.g., noM. progresivuseggs hatched, and 96% of the immature stages ofO. gossypiidied. Relative humidity is, thus, an important abiotic factor influencing the population dynamics of both species and may explain part of the decrease in populations observed in the middle to the end of the dry season and the virtual absence of mites during the wet season.