Experiments were conducted to determine the effects of relative humidity on the population dynamics of single and mixed species of Dermatophagoides farinae Hughes, D. pteronyssinus (Trouessart), and Euroglyphus maynei (Cooreman) at specific relative humidities maintained at 20 degrees C, with unlimited food. The population density of single and mixed species (D. farinae and D. pteronyssinus) increased exponentially when cultured at 65, 70, and 75% RH. The mean population growth rates were 17.3 +/- 4.4 SD and 32.5% +/- 4.7/wk for D. farinae and D. pteronyssinus, respectively. Mean population doubling times were 2.2 +/- 0.3 and 4.2 +/- 1.3 wk for D. pteronyssinus and D. farinae, respectively. Mixed species cultures, started with equal numbers of D. farinae and D. pteronyssinus, resulted in higher percentages of D. farinae than D. pteronyssinus. In cultures started with 75% of one species and 25% of the other, the more numerous species remained dominant and in similar ratios throughout the experiment. Both D. farinae and D. pteronyssinus population densities maintained at 85% RH declined over a 12-wk culture period because of mold growth. E. maynei were unable to survive at 65, 70, 75, and 85% RH, which indicated that their climatic requirements were different from those of D. farinae and D. pteronyssinus. Population densities of D. farinae and D. pteronyssinus cultures declined when held at 21-22 degrees C and relative humidities of < or = 50%; however, at 50% RH, significant proportions of the populations survived for 10 wk. Half-life for desiccation of D. farinae and D. pteronyssinus at 45% RH was 11.5 and 1.2 wk, respectively, but at 50% RH was 86.3 and 4.0 wk, respectively. The data indicated that a < or = 50% RH would have to be maintained for long periods to reduce both D. farinae and D. pteronyssinus by desiccation procedures. The results of this study show that D. farinae and D. pteronyssinus have high reproductive potentials and population growth rates, which indicate that mite reduction procedures must be thorough or mite densities will return to high levels quickly following remediation if adequate food and suitable microclimatic conditions exist.