Pseudomonas corrugata and P. mediterranea are non fluorescent Pseudomonas species that represent major groups of bacteria responsible for tomato pith necrosis. P. viridiflava is also reported as an important pathogen associated with this disease. The aim of the present study was to determine differences in abilities of P. corrugata, P. mediterranea, Pmc (a new Portuguese lineage of P. corrugate) and P. viridiflava to colonize tomato roots and stems from contaminated soil. One strain representative of each species was chosen after a characterization of a collection of Portuguese pathogenic strains using phenotypic and molecular tools (Moura, 2005). Following inoculation of the soil substrate, presence (using Bio-PCR) and population sizes of the different bacterial strains were monitored in soil substrate, root and stem fragments at regular sampling dates (3, 9, 16, 28, 37 and 49 days after inoculation). Strains of P. corrugata, P. mediterranea and Pmc were always detected in the soil substrate during seven weeks following inoculation, with a concentration decreasing from 3,44*108 to 6,97*104 ufc g-1 of substrate. The behaviour of these three pathogens was similar concerning their progression inside tomato tissues, with population sizes ranging from 9,98*105 to 1,64*104 ufc g-1 root. These three strains progressed into tomato stems ahead of the soil until the fourth internode in the absence of disease symptoms. P. viridiflava was not detected in contaminated soil substrate seven weeks after inoculation. It did not survive in the roots for a long time, and did not progress in tomato's stem. In conclusion, P. corrugata and its variant Pmc, and P. mediterranea were able to colonize tomato roots and stems from contaminated soil without plant injury. In contrast, P. viridiflava was not able to maintain detectable populations in soil neither to colonize tomato plants from contaminated soil.