Microbiological studies in a modified extensive shrimp culture system at Nambuthalai, southeast coast of India were carried out for a period of 120 days. Population dynamics and distribution profile of luminous bacteria and total heterotrophic bacteria in the water, sediment and animal samples were monitored. Luminous bacteria associated with exoskeleton, gills and gut were isolated and quantified. The total heterotrophic bacterial counts ranged from 1.3 x 10 sup(4) to 25.3 x 10 sup(4) CFU ml sup(-1) in water and 1.5 x 10 sup(6) to 26.2 x 10 sup(6) CFU g sup(-1) in sediment. The Vibrio harveyi population density varied between 0.6 x 10 sup(4) and 8.8 x 10 sup(4) LCFU ml sup(-1) in water and from 1.2 x 10 sup(6) to10.4 x 10 sup(6) LCFU g sup(-1) sediment respectively. The gut of the animal was found to harbor high density of V. harveyi than gills and exoskeleton. The total heterotrophic bacteria and V. harveyi population density showed increasing trend during the culture period. The high V. harveyi density observed in this study at the end of the culture period correlated with the outbreak of white spot disease.