Chitinase activity has been investigated in mycorrhiza-resistant (myc⁻), non-nodulating (nod⁻) isogenic pea (Pisum sativum L.) mutants in an attempt to understand the reasons for such resistance to symbiotic fungi. Activities from control and Glomus mossease-inoculated roots of myc⁻ mutants were compared to the corresponding mycorrhizal (myc⁺), nodulating (nod⁺) wildtype pea genotype cv. Frisson. A pea mutant only deficient for the nod⁻ character [myc⁺, nod⁻] was also studied. Two acidic and two basic chitinase isoforms were detected in control roots of all peas tested after native polyacrylamide gel electrophoretic (PAGE) separation of proteins at pH 8.9 or pH 4.3. No difference in basic chitinase isoforms patterns occurred in the various pea genotypes in the presence of G. mosseae. However, as soon as 1 week after infection, an additional acidic chitinase isoform was observed in extracts of G. mosseae-colonized roots from the wildtype pea genotype and the [myc⁺, nod⁻] mutant. The isozyme had a molecular mass of approximately 27 kDa, estimated by sodium dodecyl sulfate (SDS)-PAGE under non-reducing conditions, and exhibited a faint lysozyme activity by lysis of Micrococcus luteus cells. It was not detected in the [myc⁻, nod⁻] pea mutant, unless plants were grown under conditions which increased the number of appressoria. The host origin of the 27 kDa chitinase isoform was indicated by its presence after infection with another Glomus species, and from the fact that it was not detected in a mixture of germinated spores and mycelia of G. mosseae.