From houses and flats in Copenhagen inhabited by people allergic or non-allergic to house-dust, 69 samples of dust from mattresses have been collected and investigated for their content of mites and other arthropods. Three species of house-dust mites (Dermatophagoides pteronyssinus (Trt.), D. farinae Hughes, Euroglyphus maynei (Cooreman) Fain) were thus for the first time registered from Denmark besides some other mites and insects. The house-dust mites showed no preference for any of the rooms of the allergic patients nor for those of the controls (Table I). Outside the mattresses house-dust mites were found numerously below cross-bars, on wooden fillets and on floor panels beside the bed. In dust containing no skin scales living mites were not found; with air currents empty skins of mites had been transported around the room (Table 2). The house-dust mites are regarded ecologically to be the best adapted group of indoor mites to stand the low humidities (30-40 % R. H.) of centrally heated houses. Hypothetically it is proposed that the allergens of the house-dust mites are connected with the secretion of the lateral oil glands of hysterosoma.