We compared 2 1 bacterial strains isolated in Belgium from cultivated Agaricus bisporus, Pleurotus ostreatus and Psalliota edulis showing typical brown blotch symptoms, with 12 culture collection strains of Pseudomonas tolaasii, nine P . agarici strains causing drippy gill, four 'P. ginger? strains causing ginger blotch, three Pseudomonas strains responsible for mummy disease, three saprophytic P . Jluorescens, four P . aeruginosa and three 'P. reactans' strains. All strains were characterized by 147 auxanographic API tests (API 50CH, API 50A0, API 50AA) and by 128 (for 14 strains, 55) additional biochemical, serological and phytopathological features. The results were analysed by numerical methods. Taking into account also results obtained by gel electrophoresis of soluble proteins and by DNA:DNA hybridizations, we were able to differentiate seven groups, corresponding respectively to P. aeruginosa (phenon I), P.flu0rescen.s biovar I1 (phenon 11), 'P. gingeri' (phenon 111), P . tolaasii, including nine of our own isolates (phenon IV), the so-called white line reacting organisms, containing 11 of our own isolates (phenon V), two mummy disease isolates (phenon VI) and P . agarici (phenon VII). P. tolaasii formed a homogeneous group, containing both virulent and avirulent strains. The saprophytic white line reacting organisms of phenon V were, despite their phenotypic similarity, heterogeneous genotypically and in their gel electrophoresis patterns. A determinative scheme for the differentiation of P . tolaasii, the white line reacting organisms and the other Pseudomonas species occurring on mushrooms is proposed.Brown blotch disease (Tolaas, 1915), the most serious and commonly spread disease on cultivated mushrooms, is caused by Pseudomonas tolaasii (Paine, 191 9). The appearance of sunken chocolate-brown lesions on the caps and stipes of affected mushrooms is responsible for considerable quality losses. As much as 7-10% of the total Belgian mushroom production may be lost due to this disease. Three other members of the genus Pseudomonas induce diseases on cultivated mushrooms : P . agarici, causing drippy gill (Young, 1970); 'P. gingeri', causing ginger blotch (Wong et a/., 1982) and an unidentified Pseudomonas species responsible for mummy disease (Schisler et a/., 1968). Moreover, the occurrence of many saprophytic pseudomonads has been reported frequently on mushrooms and in their environment (Olivier et al., 1978; Zarkower et al., 1984). Some of these saprophytic Pseudomonas were capable of inducing a 'white line reaction' with P . tolaasii and were designated as 'reacting organisms' (Wong & Preece, 1979) or 'inducers' (I strains) (Zarkower et al., 1984). Few authors have discussed the taxonomic position of Pseudomonas strains associated with mushrooms. From the existing literature (Young, 1970;Fahy, 1981), the differentiation of P . tolaasii and P. agarici was possible, whereas differentiation between the other pseudomonads associated with mushrooms posed more problems. According to Fahy (1981) P. tolaasii...