We tested the hypothesis that failure to establish symbiosis with vesicular-arbuscular (VA) mycorrhizal fungi is correlated with glucosinolate concentrations in Brassica, a representative genus of the Capparales. Brassica campestris and B. napus cultivars (brassica) with a range of glucosinolate concentrations (7–524 μmol g1 f. wt in roots) were grown together with the VA mycorrhizal fungi Glomus mosseae and Gigaspora gigantea in agar. Fungal growth was observed in situ but fungi did not penetrate brassica roots. However, normal germ tube growth of the VA mycorrhizal fungi occurred near brassica roots when compatible hosts grew nearby. These compatible hosts developed normal mycorrhizal infections. These results suggest that brassica roots do not produce a diffusable inhibitor of VA mycorrhizal fungi, but lack a diffusable growth stimulus present near roots of compatible hosts.