Tomato spotted wilt virus (TSWV) is a causal agent of a devastating disease of many crops including tomato in Iran, but there is not enough information on disease control especially by using resistant cultivars. Twenty isolates of TSWV collected in North-east of Iran (Mashhad area) were evaluated for symptom severity. One severe isolate (M-5) was selected because of the distinct differences in its virulence and symptomatology on tobacco. M-5 isolate produced severe systemic necrosis, deformation and a lot of local lesions whereas the other isolates caused a few chlorotic spots and mild systemic symptoms. Distinct differences in the response of 10 selected commercial cultivars of tomato to this isolate were observed. BHN 555, showed a resistant reaction, whereas the other tomato cultivars; Cheff, Early urbana-y, Primo, Fallat III, Kingstone, Kal G early, Petopride-2, Petropride-5 and Super-H were susceptible to M-5 isolates. More isolates of TSWV and commercial cultivars of tomato should be studied to screen for most resistant ones against existed TSWV isolates in this geographic area and also for other regions of Iran.