Technical Abstract: The resistance phenotypes of two potato cultivars to two isolates of Alternaria solani, causal agent of early blight, were studied under greenhouse conditions. The two isolates contain varying degrees of aggressiveness on both susceptible and resistant phenotypes of potatoes. A bioassay was used to study the role of liquid culture exudates produced in vitro in pathogenicity and elicitation of disease symptomology in seedlings as well as detached leaves. Responses of host genotypes to the exudates of the two A. solani isolates were significantly different. Interestingly, the phytotoxicity effect of the culture filtrate from the more aggressive isolate A was higher than from isolate N suggesting an increased concentration or strength of the toxins produced. Furthermore, the resistant cultivar ‘Diamond’ elicited fewer symptoms to each isolate when compared to the susceptible cultivar ‘Granula’.