Abstract Chitosans which had been: (1) derived chemically from the chitin of fungal cell walls, (2) accumulated in Fusarium solani/pea interactions or (3) released from chitinase and β-glucanase digestion of sporelings, were used to determine if these fungal polymers had the biological activity of the chitosan chemically derived from crustaceans. The biological activity of the cell wall chitin-derived chitosan from F. solani f. sp. phaseoli mimicked that of shrimp chitosan and was somewhat superior to that from f. sp. pisi. F. solani f. sp. phaseoli chitosans inhibited germination of F. solani macroconidia at concentrations as low as 8 μg ml −1. 100 μg ml −1 of this chitosan provided protection against F. solani f. sp. pisi in pea pod tissue for periods of at least 5 days while 10 μg ml −1 could only maintain resistance for up to 3 days. In comparisons of chitosan-like oligomers released from the ff. sp. pisi and phaseoli, greater proportions of [ 3H]-N-acetylglucosamine labelled chitosan fraction (heptamer or larger) could be recovered both from f. sp. phaseoli/pea interactions and from f. sp. phaseoli germlings in contact with a chitinase and β-glucanase-rich basic pea protein fraction. The results indicate that the chitosan heptamer-plus fraction readily recovered from these plant-fungal interactions is able to function as a major biological signal in pea/- Fusarium interactions while the chitosan pentamer which preferentially accumulates in f. sp. pisi had less detectable biological activity.