Five cultures of microalgae (Chlorella minutissima, Tetraselmis chui, Nannochloropsis sp., Arthrospira platensis and Isochrysis sp.) with no culturable bacteria were tested for their ability to inhibit the growth of six Vibrio bacterial strains (V. parahaemolyticus, V. anguillarum, V. splendidus, V. scophthalmi, V. alginolyticus and V. lentus). The influence of light on the antibacterial activity of the microalgae was investigated. All microalgae cultures inhibited the growth of bacteria compared with the control treatments (P < 0.05), and their antibacterial activity was not influenced by the presence or absence of light. In the control groups, the numbers of bacteria increased exponentially during the experimental period in the absence of microalgae cells demonstrating that the bacterial cells were able to utilize the growth medium of microalgae cultures. The present results may explain the low levels or absence of Vibrio strains in microalgae cultures, and the positive effect of addition of microalgae in rearing of fish larvae, and implicate the production of antibacterial compounds by microalgae cells.