Supplemental substrates for mushroom cultivation media are potent sources for fungal contamination of indoor environment in cultivation houses. The present study was performed to investigate the presence of harmful fungi in the supplemental materials (beet pulp, cotton seed meal, kapok meal, peat moss, wheat straw) imported from China, Egypt, Germany, Indonesia and Pakistan. Fungal concentration in the supplemental materials ranged from 1.3×102 to 1.8×103 cfu/g. A total of 11 genera and 21 species were identified. Two genera and two species were found in Germany peat moss. In Pakistan wheat straw, three genera and four species were detected. In Egypt beet pulp and China cotton seed meal, four genera and five species and seven genera and ten species were shown, respectively. Among the identified fungi, Acremonium sp., Aspergillus chevalieri, Cryptococcus adeliensis, Eurotium amstelodami, Paecilomyces variotii and Rhodotorula mucilaginosa were known as human pathogens. This study provided evidence that pathogenic yeast and filamentous fungi are present in imported agricultural byproduct materials for mushroom cultivation. Keywords : Culture media, Harmful fungi, Indoor environment, Mushroom cultivation house, Supplemental substrates