Within limits, increasing agitation rate in continuouscultures of microalgae enhanced biomass productivityby reducing the length of the continuous dark period (orlight-limited period) experienced by the cells. Both P.tricornutum and P. cruentum experienced various levelsof cell damage at high agitation rates. Compared to P.tricornutum, the microalga P. cruentum had a highertolerance for the otherwise damaging forces. The causeof the cell damage was not mechanical agitation per se,but the rupture of small gas bubbles at the surface of theculture. Increasing agitation rate reduced the bubble sizeto produce a more damaging environment than thatwhich existed when the bubbles were larger. Addition ofPluronic F68 over a concentration range of 0.01–0.10%protected cells in a concentration-dependent manner. APluronic concentration of 0.1% appeared to affordcomplete protection to P. cruentum und er conditionsthat had caused severe damage and culture washout inthe absence of Pluronic. These results lead to the fol-lowing specific conclusions