Penicillium species, well-known indoor airborne fungi, are considered to he important causative agents of extrinsic bronchial asthma. For selecting the proper species for clinical investigations, it is important to know which are found indoors. tn the present study, 176 plates of modified Mehrlich's Medium were exposed for 20 min in living rooms and bedrooms of 88 homes in Taipei, Taiwan. A total of 5897 colonies of fungi were isolated, varying from one to 329 per house. Of the total, 910 colonies (15.4%) were identified as Penicillium spp. The three most frequently encountered species were P. citrinum, P. crustosum and P. implicatum. Penicillium citrinum accounted for 40.5% (369 from 28 houses); P. implicatum, for 5.2% (47 from three houses); and P. crustosum, for 2.1% (19 from seven houses). Penicillium citrinum was judged to be the most prevalent airborne fungus in Taiwan. The allergenic significance of this mold is under further investigation.