By adjusting pH through additions of NaOH solution (~.8N), the response of recently metamorphosed Macrobrachium rosenbergii postlarvae to pH values ranging from 7.5 to 12.0 was determined. At pH 9.0 weak animals and subsequent mortalities were observed. At pH 9.5 or greater, mass mortalities occurred. The effect of high pH resulting from algal photosynthesis in water with a heavy phytoplankton bloom was also evaluated. High mortality rates were recorded in water with a heavy bloom and high pH (10.0-10.5) compared with control water without a bloom and with a lower pH (8.5-9.0).