The microalga Haslea ostrearia is known as a diatom with the peculiarity of producing a water soluble blue-green pigment called marennine. This pigment has antibacterial activities against pathogenic bacteria of bivalves produced in aquaculture. The present study evaluates the effect of culture supernatant of the diatom Haslea ostrearia containing marennine on the clearance rate (CR) and survival rate (SR) of adult Pacific oyster Crassostrea gigas infected with the bacterial pathogen Vibrio aestuarianus. The greening of oysters was conducted by exposing bivalves to 0.5 mg L-1 of marennine for three days, and the effect of marennine on the CR and SR of oysters was evaluated by comparing four groups of treatment: control, greened, control+infected and greened+infected. The SR of oysters in each treatment was monitored for 11 days. Our results showed that marennine concentration decreased with time and dropped significantly on the last day of greening, and that marennine at an initial concentration of 0.5 mg L-1 did not significantly affect the CR of oysters in all treatments. Mortality of oysters was only observed in the control+infected group, but it was not significantly different from the control. Hence, the present study showed that the greening using 0.5 mg L-1 of marennine did not affect SR of adult oyster C. gigas, but further studies are needed to determine whether H. ostrearia supernatant could be used as an antibacterial and prophylactic agent in bivalve aquaculture