The predominant bacterial genera present in the fish tank water of a sustainable tilapia fish farm system were found to be Vibrio (39.7-69%), Aeromonas (16-21.6%), Pseudomanas (1.8-2.0%), and Shewanella (4.0-9.9%). Each of the 69 Vibrio isolates obtained on March 5, 2003 from tilapia fish tank water was subjected to RAPD-PCR analysis separately with three decamer primers PB1, PB4 and HLWL74. Based on the analysis of band patterns, the 69 Vibrio isolates yielded 32 RAPD types with primer PB4, 40 with primer PB1, and 38 with primer HLWL74 respectively. There were 67 composite RAPD profiles derived from the 69 Vibrio isolates. A total of 9 genomic Vibrio clusters were identified. These results indicated that a considerable genomic heterogeneity existed among the Vibrio isolates and that no one clone exerted numerical dominance among these Vibrio isolates.