The prevalence of mould genera and their association with the house dust mite (Dermatophagoides pteronyssinus) were investigated in the homes of 192 asthmatic patients and 125 healthy people in a Romanian town with a relatively high degree of humidity. The dominant moulds which developed in the culture media exposed on the bedroom floors were Cladosporium, Penicillium and Aspergillus. Living Dermatophagoides pt. and their eggs were present mostly in the colonies of Cladosporium; in those of Penicillium the house dust mites were dead and disintegrated. It is concluded that allergy to Dermatophagoides pt. must not be connected to house dust as such but to the presence in the dust of certain mould spores.