In order to study the antibacterial activities of chitosans obtained from cuticle of housefly (Musca domestica) larvae and affecting factors, chitosans with different molecular weights prepared from cuticle of housefly larvae were examined against six bacteria at different conditions. The results showed that chitosans  with the molecular weights of 21－251 kD had stronger antibacterial activities. Chitosans showed higher antibacterial activities at low pH value. The minimum inhibitory concentration of chitosans ranged from 0.03%－0.06%. Ca²⁺ and Mg²⁺ could reduce the antibacterial activity of chitosans remarkably. Molecular weight of chitosans, pH value and ion were the factors strongly affecting the antibacterial activities of chitosan, which was also significantly related to the strain of the bacteria. Chitosan could increase  cell membrane penetrability of the bacteria and cause the leakage of cytoplasm.