Macrobrachium rosenbergii ponds at the Centre Océanologique du Pacifique are subject to increases in pH exceeding 10.5 during growing experiments. The high levels of pH are responsible for mortality of prawns, especially when molting. The photosynthetic activity of dense phytoplankton blooms are primarily responsible for high pH. The algicide Clarosan (CIBA-Geigy) was used to thin phytoplankton blooms. Experiments showed that treatments with 0.02 mg/liter result in a rapid fall in the pH. At this dose Clarosan is not deleterious to the prawns. [NOT CONTROLLED OCR]