A total of 100 tomato plant samples exhibiting typical symptoms of bacterial canker were collected from tomato-growing areas of north-western areas of Pakistan. On isolation of the causal agent of this disease, 47 out of 100 samples yielded typical colony morphology of Clavibacter michiganensis subsp. michiganensis, when grown on Nutrient agar (NA) medium. 34 isolates out of the 47 exhibited Clavibacter michiganensis subsp. michiganensis like cultural characteristics when grown on Yeast extract-Dextrose-CaCO3 (YDC) medium. However, 27 isolates only out of these 34 were confirmed to be Gram positive. Pathogenicity of the 27 Clavibacter michiganensis subsp. michiganensis isolates were confirmed using cotyledon, young seedlings and old seedlings assays. Results showed that all these isolates were able to cause bacterial canker disease symptoms on tomato plants. Cotyledon assay was negative for isolates from Mansehra (MNS2); whereas, MNG1 isolate didn't produce any symptoms for all three tests. Old seedlings test was negative for four isolates mainly; MKD2, MNG1, DIK1 and NWS2. To investigate the hypothesis that appearance of symptoms depends on inoculum concentration of pathogen; two aggressive isolates of Clavibacter michiganensis subsp. michiganensis i.e., MKD1 (from Malakand) and KLM1 (from Kalam) were inoculated into tomato plants at different concentrations (i.e. 1×102 - 108 cells/ ml). Results reported that earlier and more severe disease symptoms of bacterial canker were produced by the more aggressive isolate (MKD1); compared to the moderately aggressive isolate (KLM1), which produced milder and delayed disease symptoms on tomato plants.