Two components of the greenwater system, the tilapia water and Chlorella, were investigated separately for their effect on the population of luminous bacteria. For the investigation of tilapia water, Tilapia hornorum was stocked at different biomasses 0, 1 and 3 kg/10 m³. Vibrio harveyi was introduced to the tanks at a final concentration of 10³ cfu/ml. Luminous bacteria could not be detected in tanks stocked at 3 kg/10 m³ from day 4 to day 6, and on day 6 in tanks stocked at 1 kg/10 m³. The bacteria could still be recovered in tanks without tilapia on day 6. Investigation of the effect of chlorella alone on V. harveyi was undertaken using sterile flasks containing 500-ml autoclaved seawater provided with aeration. In treated flasks Chlorella was added, whereas no Chlorella was added to control flasks. No luminous bacteria were recovered on day 2 and day 3 in flasks with Chlorella, while those without the microalgae still harboured luminous bacteria at day 3.