Between April 1972 and March 1973, 190 samples of house dust from areas in northwestern Switzerland were examined for the presence of mites. During autumn, 85% of the samples contained mites; during winter, only 50%. Thirty-eight species were identified, 32 for the first time in Switzerland. Of the approximately 11,000 mites isolated, Dermatophagoides pteronyssinus comprised 70.7% of the total, Euroglyphus maynei 17.7%, Dermatophagoides farinae 5.7%, Glycyphagus destructor 2.0%, Glycyphagus privatus 0.8% and various other species 3.1%. The mite fauna in bedding and cages of domestic animals such as dogs, cats, hamsters, guinea pigs and singing birds was determined. The numbers support the concept that patients who are seemingly allergic to tlieir pets might in reality be allergic to the mites associated with the pets' living quarters. The morphology of the immature stages of D. pteronyssinus, D.farinae and E. maynei was studied with the aid of the phase contrast and the scanning electron microscope (SEM). Measurements for the 3 main species and a key for their identification are provided. In E. maynei, male and female tritonymphs were distinguished.