Abstract: It is well known that Chitosan has no toxicity to animals. Amazingly, the chitosan is poisonous to the loach（Misgurnus anguillicaudatus）and trigger acute death. The cause of death is worth investigating due to chitosan is a food additive. Loaches were treated with different kinds of chitosan or different concentration chitosan or water soluble chitosan at different temperature. The half lethal concentration (LC50), mucilage of loach shedding rate, respiration rate, dissolved oxygen concentration, ion and ammonia permeability, erythrocyte cell nucleus abnormality were measured. Results showed that all chitosan caused loach rapid death. The higher concentration or the higher temperature, the earlier loach died. LC50 of chitosan for loach was showed as 16 mg/L. The mucilage of loach was striped by chitosan and there was a lot of adhesion material blocking fish gill. When loach was treated by chitosan, the respiration rate increased significantly, the decrease of pH value in loach after the death was retarded, the exudation of chlorine, sodium, potassium ion and ammonium were promoted, and the ammonia content exceeded the tolerance concentration in the end. Therefore, chitosan destroyed mucilage of loach, led to turbid liquid, promoted the exudation of cell ions and ammonia, and destroys the biological function of cells. It inhibited loach skin respiration and gill respiration, stimulated loach nerve and an Irritable swimming, that depleted glycogen in loach at early stage, led to loach unable swim to surface of water for intestinal respiration at later stage. Greatly increasing of respiration rate of loach, resulted in dissolved oxygen in water was below the safely limit. Chitosan harm loach respiratory system and leads it to death, remind us that if there are other factors cooperate with chitosan into human cells or inhibit pepsin activity which can enzymatic hydrolyze chitosan, may promote toxicity of chitosan to the human body.

Key words: chitosan, loach, death, toxicity