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Three types of chitin as α (exoskeleton of crustaceans), β (cuttlebone of cephalopods) and γ (some fungal cell wall) exist in nature. Chitosan is the most important derivative of chitin, which has various biological activities. In this study, α, β and γ-chitosans were extracted from Portunus segnis exoskeleton, Sepia pharonis cuttlebone and Aspergillus niger cell wall, respectively. Samples of crab and sepia from Bandar Abbas coastal waters were gathered in the summer of 2014 and samples of Aspergillus niger (PTCC 5223) were obtained from the Iranian Research Organization for Science and Technology (IROST). The amount of chitosan extracted from the Crab was significantly higher than other samples. Antimicrobial properties of these three types of chitosan were explored against 9 bacterial and 2 fungal strains using disc diffusion method. On average α chitosan and γ chitosan revealed to have highest and lowest antimicrobial activities, respectively. Antibacterial properties of chitosan extracts were more than their antifungal properties. Gram negative bacteria as well as gram positive bacteria were sensitive to three types of chitosan. According to observed results, all three types of chitosans have good antimicrobial activities, and further investigations are suggested.