The host range of the soilborne obligate biotroph, Plasmodiophora brassicae was investigated. Evidence is presented that infection by P. brassicae might occur in non-Brassica species, leading to the potential formation of resting spores. Structures resembling P. brassicae were found in the root cortex of Tropaeolum majus, Carica papaya, Reseda alba and Beta vulgaris as demonstrated by scanning electron microscopy. Inoculation of Brassica rapa roots with spores extracted from either T. majus or B. vulgaris roots which had been previously inoculated with P. brassicae led to development of clubroot in the roots of B. rapa. It was also shown that the development of the symptom might be correlated with glucosinolate content, although other host factors are implicated in the B. vulgaris interaction with P. brassicae. In the glucosinolate-containing non-Brassicas, T. majus and C. papaya, the concentrations of benzylglucosinolate increased markedly in roots inoculated with P. brassicae, compared with the controls. There were also increases in concentrations of benzylglucosinolate in leaves of T. majus after P. brassicae infection. However, in R. alba roots, the total glucosinolate content decreased after inoculation with P. brassicae compared with the controls. High root concentrations of 2-OH-2-phenylethylglucosinolate (glucobarbarin) compared with low root indole glucosinolates in this species might limit P. brassicae infection and development. The importance of our investigations in relation to cultivation of non-Brassica species on fields infested with P. brassicae is discussed.