Supplemental substrates for mushroom cultivation media are potent resources for bacterial contamination of the indoor environment in cultivation houses. This study was carried out to investigate the presence of harmful bacteria in supplemental materials (beet pulp, cotton seed meal, kapok meal, peat moss, wheat straw) imported from Egypt, China, Indonesia, Canada, and Pakistan. Bacteria belonging to Firmicutes, Actinobacteria, Gamma proteobacteria, and Bacterioides were identified based on 16S rDNA sequence analysis. Seven genera and eight bacterial species were detected in peat moss. Four genera and seven bacterial species were found in both beet pulp and wheat straw. In cotton seed meal and kapok meal, three genera and thirteen bacterial species and two genera and six bacterial species were shown, respectively. Among the identified species, Bacillus anthracis, Bacillus oleronius, and Lysinibacillus fusiformis are known as human pathogens. Overall, the results of this study provided evidence that the imported supplemental materials for mushroom cultivation media are contaminated with pathogenic bacteria. Keywords : Culture media, Harmful bacteria, Indoor environment, Mushroom cultivation house, Supplemental substrates