Two American isolates and three Indian isolates ofFusarium vasinfectum representing two strainal groups were studied on their congenial and uncongenial hosts for their pathogenicity andin vivo production of pathogen factors. The Indian isolates produced a characteristic vein clearing symptoms on their congenial hostGossypium arboreum while the American isolates produced yellowing and abscission of the leaves on their congenial hostG. hirsutum. In cross inoculation studies these isolates produced atypical symptoms on their uncongenial hosts. Studies onin vivo production of pectin methyl esterase (PME) showed that this enzyme may have a role in pathogenesis ofG. arboreum but not ofG. hirsutum. However the enzyme polygalacturonase (PG) appears to have role in the pathogenesis ofG. arboreum as well asG. hirsutum. The toxin fusaric acid was found inG. arboreum plants infected by the Indian isolates. No fusaric acid could, however, be detected inG. hirsutum infected by any of the isolates studied. Thus fusaric acid appears to have a role in the selective pathogenicity of the Indian isolates and also its production seems to be selective. The present study indicates that different pathogen factors are involved in the development of different symptoms on the two species of cotton.