The distribution and abundance of house dust mites, Dermatophagoides spp., were studied in August, October, and December, 1975 in 4 different climatic zones in southern California. During these months, a total of 15 houses were sampled in each climatic zone. Ninety-three percent of the coastal houses were infested with mites, where D. pteronyssinus (Trouessart) dominated (78%) over D. farinae Hughes. Sixty percent of the Riverside (inland valley) houses sampled were infested, where D. farinae was dominant (67%). A 3rd species, D. evansi Fain, Hughes and Johnson, commonly occurring in birds' nests, was found once in a coastal and Riverside house. Densities of both D. pteronyssinus and D. farinae were considerably higher in coastal than in Riverside houses. Live mites were not found in the lightly mite infested houses sampled in the desert (54% positive) and mountains (27% positive). Relative humidity, which varied in houses located in different climatic zones of southern California, was noted to be the principal limiting factor influencing the distribution and abundance of D. pteronyssinus and D. farinae in these zones. Temperatures did not appear to be an important factor influencing distribution and abundance of these mites in the study zones.