Occurrence and characteristics of infectious fungi and physiologically abnormal Paecilomyces tenuipes were examined. These unfavorable fungi are recently known to cause yield loss, decreased quality of products in artificial cultivation farms of P. tenuipes. Total 529 fungal isolates were collected from 22 farms of 7 provinces. These isolates were composed of physiologically abnormal P. tenuipes strains and other infectious fungal strains, which constituted 23.2% and 50.9%, respectively. Physiologically abnormal P. tenuipes strains showed irregular synemata, and absence of local color. They were divided into 4 types or C-l, C-2, C-3 and C-4. Other infectious fungal strains were identified to 5 species, Beauveria bassiana, Fusarium sp., P fumosoroseus, Tricothecium roseum, Aspergillus parasiticus, which were 22.1, 13.8, 7.6, 4.0 and 3.4%, respectively. All of them were hyphomycetous fungi, did not produce synemata, and revealed faster growth rate than that of P. tenuipes.