Quantitative and qualitative analyses of bacterial flora associated with larval rearing of the giant freshwater prawn, Macrobrachium rosenbergii, along with important water quality parameters, were carried out over a larval cycle. Total viable counts (TVC) varied between 1.1 plus or minus 0.6 x 10 sup(4) and 9.8 plus or minus 1.5 x 10 sup(6) colony forming units (cfu) ml sup(-1) in water, 2.4 plus or minus 0.4 x 10 sup(5) and 8.6 plus or minus 1.6 x 10 sup(6) cfu g sup(-1) in eggs and 2.5 plus or minus 1.4 x 10 sup(4) and 1.6 plus or minus 1.0 x 10 sup(8) cfu g sup(-1) in larvae. The bacterial population of eggs and larvae decreased with the age. Physico-chemical characteristics of water fell within the optimum ranges. The bacterial flora was predominantly Gram-negative, comprising more then 75% of the total isolated strains. Aeromonas, Alcaligenes and Pseudomonas were the most frequently encountered genera (prevalence > 10%) in water, whereas Alcaligenes. Enterobacteriacae, Pseudomonas and Streptococcus were the most abundant strains associated with larvae. Vibrio spp. were detected in eggs and water but were conspicuously absent in larvae. The bacterial flora of eggs was similar to that of water, while a distinctly different population was observed in the larvae.