1. The effects of chitosan (CS) derived from the exoskeleton of the shrimp Macrobracium rosenbergii on bodyweight, plasma lipid profile, fatty acid composition, liver lipid peroxide (LPO) levels and plasma levels of glutamate pyruvate transaminase (GPT) were determined in normocholesterolaemic (NC) and hypercholesterolaemic (HC) Long Evans rats. 2. The NC rats were fed a diet containing 2% CS and the HC rats were fed a diet containing 2 and 4% CS for 8 weeks. Chitosan significantly reduced bodyweight gain only in HC + 4% CS rats compared with HC rats, but not in NC + 2% CS or HC + 2% CS rats. 3. Chitosan reduced plasma total cholesterol in the HC + 2% CS, HC + 4% CS and NC + 2% CS rats; however, low density lipoprotein-cholesterol decreased only in the first two groups. High-density lipoprotein-cholesterol (HDL-C) increased in the HC + 4% CS rats by 24% compared with the HC + 2% CS group and by 30% compared with HC rats; however, HDL-C did not increase in the NC + 2% CS group compared with NC rats. The level of plasma triglycerides decreased significantly only in HC + 2% CS rats compared with HC rats. 4. Chitosan significantly decreased plasma levels of arachidonic acid in the HC + 2% CS and HC + 4% CS groups, with a concurrent increase in the molar ratio of total unsaturated fatty acid (TUFA) to total saturated fatty acid (TSFA). 5. Moreover, CS increased liver LPO levels without affecting plasma levels of GPT. Liver LPO levels were positively correlated with the TUFA/TSFA molar ratio. 6. The present study suggests that dietary CS decreases the atherogenic lipid profiles of both NC and HC rats and reduces the bodyweight gain of HC rats.