House-dust samples from beddings and floors of house-dust sensitive patients in southern California were collected with a house-dust concentrator attachment. House-dust mites (Dermatophagoides spp.) were washed and isolated from the dust using the volumetric washing technique. In 10 houses Dermatophagoides abundance on the surface of mattresses encased with plastic was significantly lower than abundance on comparable nonencased mattresses at the 5% probability level. Sequential vacuuming (3 times) of hard-surface flooring and carpeting near beds removed mites completely from the smooth surface but not from the carpeting. The abundance of mites on the floor under plastic-encased bedding was significantly lower (at 5% level) than on the floors under comparable nonencased bedding in the same houses. The average age of heavily mite-infested houses was 27 years; houses with light infestations (10 mites/ml dust) were 14 years old on the average. Heavier Dermatophagoides mite infestations were found in bedrooms sleeping more individuals on a regular basis. Higher levels of infestation were associated with observably poorer levels of housekeeping and sanitation. A 25-year-old mattress, infested heavily with house-dust mites, was sequentially surface-vacuumed in the laboratory and, after such vacuuming, the cloth covering and fiberfill were analyzed for mite density, using our standard volumetric washing technique. Four sequential vacuumings of the same area removed 9% of the dead mites, 4.5% of the living mites. A study of the spatial distribution of the Dermatophagoides population showed the highest densities at or near the mattress cording and at the surface of the fiberfill, suggesting that the living mites are confined to or close to the surface of the mattress fiberfill and cloth covering.