Abstract: Research studies were conducted in the green house to investigate the nature of pathogenicity of Alternaria solani, 5 isolates on two tomato species Lycopersicon esculentum and L. esculentum x L. hirsutum F1 hybrids and to describe the procedures by which it would be possible to determine the 5 isolates of Alternaria pathogenic action and progressive disease development. The results showed that the five isolates of Alternaria solani were non-pathogenic against the two tomato species tested in the experiment.