Potato diseases are one of the major factors limiting the production of the crop in most parts of the world. The diseases caused by fungi are the most important and damaging diseases of the potato. Plants have different mechanisms for self-defense against pathogens. Primary defensive responses are known as base resistance. Various types of cellular, molecular and biochemical mechanisms, along with different signal transduction pathways, are involved in this resistance. The Alternaria alternata causes brown spots in the potato leaves. In this study isolates of A. alternata were identified in potato fields of Isfahan and pathogenicity of two aggressive isolates on nine common potato cultivars (Including Agria, Arinda, Spreete, Marfona, Milva, Sante, Satina, Jelly, and Bourne) were investigated. In the pathogenicity test on forty-five days potato plants, two cultivars of Sante and Agria were identified as resistant and susceptible cultivars to this pathogenic fungus respectively. To investigate some of the defense strategies involved in base resistance in potato A. alternata pathosystem, the role of antioxidant enzymatic systems including peroxidase (POX) and catalase (CAT), as well as total phenol content in susceptible (Agria) and resistant potato cultivars (Sante) were evaluated 60, 120, 180 and 240 hours after inoculation with A. alternata. The results indicated that the low activity of the POX and CAT enzymes and the total phenol content in the resistant cultivar (Sante) compared with the sensitive cultivar (Agria), which could be due to the micro-morphological characteristics of the leaf area, which does not allow the fungus to enter the plant tissue. Accordingly, the classification of genotypes into resistant and susceptible cannot be evaluated solely on the basis of host enzyme activity and the way of host's response reaction to the pathogen should also be taken into consideration.