Quantification and characterization of bacterial flora in the hepatopancreas (hp) of pond-reared Penaeus monodon juveniles affected with luminous bacteria were conducted in 1994 and 1995. Shrimp samples were taken from 23 grow-out ponds, 14 of which had disease outbreaks. Luminous bacterial (LB) load of the shrimps' hp with (mean=2.4×101 colony forming units (CFU)/hp) and without (mean=0.3×101 CFU/hp) disease outbreaks were comparable during the first 15 days of culture (DOC). During disease outbreaks at 18 to 32 DOC, however, LB load of affected shrimps (mean=9.0×104 CFU/hp) were higher than healthy shrimps (mean=7.0×101 CFU/hp). At 50 to 60 DOC, levels of LB were comparable in older shrimps with or without disease. Total viable and presumptive Vibrio counts were also comparable in both shrimp samples from 1 to 60 DOC. Characterization of the 172 bacterial isolates collected showed that most (90.12%) were Vibrio species dominated by V. harveyi (27.91%), V. splendidus II (13.37%) and V. parahaemolyticus (10.46%).