The antifungal mode of action of chitosan has been studied for the last 30 years, but is still little understood. We have found that the plasma membrane forms a barrier to chitosan in chitosan-resistant but not chitosan-sensitive fungi. The plasma membranes of chitosan-sensitive fungi were shown to have more polyunsaturated fatty acids than chitosan-resistant fungi, suggesting that their permeabilization by chitosan may be dependent on membrane fluidity. A fatty acid desaturase mutant of Neurospora crassa with reduced plasma membrane fluidity exhibited increased resistance to chitosan. Steady-state fluorescence anisotropy measurements on artificial membranes showed that chitosan binds to negatively charged phospholipids that alter plasma membrane fluidity and induces membrane permeabilization, which was greatest in membranes containing more polyunsaturated lipids. Phylogenetic analysis of fungi with known sensitivity to chitosan suggests that chitosan resistance may have evolved in nematophagous and entomopathogenic fungi, which naturally encounter chitosan during infection of arthropods and nematodes. Our findings provide a method to predict the sensitivity of a fungus to chitosan based on its plasma membrane composition, and suggests a new strategy for antifungal therapy, which involves treatments that increase plasma membrane fluidity to make fungi more sensitive to fungicides such as chitosan. © 2010 Blackwell Publishing Ltd.