Citrus blast disease caused by Pseudomonas spp is among the most injurious diseases of citrus. in the northern citrus growing provinces of Iran. To determine the species and pathovars of Pseudomonas strains associated with the disease in Mazandaran province and in the neighboring areas in Guilan and Golestan provinces and as well to assess their phenotypic heterogeneity and virulence, symptomatic samples were collected from the affected trees during 2009-2010. Among the strains isolated from the leaves and twigs, 115 strains appeared, in preliminary testing to be pathogenic to sour orange (Citrus aurantium) leaves. Based on biochemical properties, the isolates were grouped into 16 separate categories. Strains were positive in tests for hypersensitive reaction in tobacco, negative in production of oxidase but varied in the production of levan, arginine dihydrolase and rotting of potato tuber slices. Some isolates despite their ability to rot potato tuber slices, were not sufficiently similar to P. viridiflava to be called as such. Based on their phenotypic characteristics, strains of group XII to XVI (39% of total isolates) were identified as Pseudomonas syringae and those of group XI (3% of total isolates) as P. viridiflava. Further studies are needed to clarify the taxonomic status of the remaining strains which appeared distinct from Pss and P. viridiflava. The virulence of the strains was evaluated through inoculation of each strain onto sour orange, sweet orange (C. sinensis, Washington navel) and alemow (C. macrophylla) leaves. There were noticeable differences observed in the size of lesions produced by different strains, on the basis of which basis the isolates of either pathogen could be differentiated into weak, moderate, or highly virulent strains.