This paper aims to study the seasonal prevalence of Aeromonas hydrophila in naturally collected and cultured O. niloticus.Results showed that prevalence of the infection was higher in cultured fish during the summer season than in wild fish. The observed clincial signs in the examined fish suffering from Motile Aeromonas Septicemia (MAS) were varied septicemia, ascitis, eroson, ulceration, detachment of scale and exophthalmia. The post mortem findings varied from congestion to focal lesions in the liver, spleen, and kidney. 25 isolates of A. hydrophila from sterile-intestinal organs of naturally infected Nile tilapia were collected. There was a pronounced variation in both the biochemcial and enzymatic profiles within the motile aeromonas species. Variation was evident in the hemolytic activity. Similarly, there was variation in pathogenicity among the 25 isolates of A. hydrophila. The relation between pathogenicity and the biochemical activity is discussed.