Eighty-six isolates of Erwinia salicis, the causal agent of watermark disease of willow, were obtained from culture collections and collected from various willow plantations in south-east England. These isolates were characterized by their reaction in a competitive enzyme-linked immunosorbent assay, by enzyme electrophoresis, and for lysis by a panel of five bacteriophage. Eighty-one isolates had similar characteristics and clustered at a level of similarity of at least 60%. Five isolates were found to have less than 20% similarity to the other E. salicis. Antiserum raised against E. salicis (NCPPB 2535) was found to be specific to E. salicis and did not cross-react with other bacteria tested. No geographical clustering of electrotypes was found: however, Dutch and English isolates could be distinguished by bacteriophage typing. The local distribution of electrotypes collected from diseased trees suggests that the bacterium is spread via the propagating material and that tree to tree spread is rare.