The addition of fungal cell-wall homogenates of Penicillium notatum and Cylindrocladium spathiphylli markedly stimulated the accumulation of tolytoxin, an antifungal secondary metabolite, in cultures of the cyanobacterium Scytonema ocellatum Lyngbye ex Bornet et Flahault. Evaluation of polysaccharides, proteins, and other polymers established that a limited range of polysaccharides, especially chitin and carboxymthylcellulose, selectively elicited enhanced tolytoxin accumulation in S. ocellatum. The elicitor activity of fungal cell-wall preparations could be correlated with the chitin content of the preparation. Polymeric chitin was half-maximally effective at a concentration (EC50) of 19 mg.L-1, whereas chitin oligoments were more effective (EC50= 3.3 mg.L-1) in eliciting enhanced tolytoxin accumulation. The elicitor activity of either purified chitin or an elicitor-active fungal cell-wall preparation could be destroyed by treatment with chitinase. The results suggest an ecological role for tolytoxin as an inducible chemical defense agent (phytoalexin) capable of protecting S. ocellatum against fungal invasion.