Bacteria and fungi associated with the house-dust mite Dermatophagoides were investigated. The house-dust mites were washed with sterile distilled water and homogenized. Bacterial count, isolation and identification of the bacteria and fungi were made with routine laboratory methods for medical pathogens. The number of bacteria counted per 100 mites in positive samples was 12.7 for laboratory-bred Dermatophagoides pteronyssinus and Dermatophagoides farinae, more than 121 for mites from control houses, and more than 66 and 121 for those from houses with asthmatic and Kawasaki disease patients, respectively. The bacteria isolated from the house-dust mites were Bacillus spp., Staphylococcus spp., Gramnegative non-fermenting rods and Gram-positive coryneform rods. As for fungi, Aspergillus spp., Penicillium spp., Cladosporium spp., Alternaria spp., Acremonium spp., Paecilomyces spp. and yeasts were isolated. All of those microorganisms found were indigenous microbes and there was no particular difference in their occurrence between the house-dust mites from control houses and those from houses with asthmatic or Kawasaki disease patients. The public health importance of the house-dust mites as a possible vector of the microorganisms was discussed.