Lime improves conditions for plant growth, thereby providing a greater food base for fish production. Because there are few data on the effects of lime on aquatic plants, this investigation was undertaken to gather information on phytoplankton density and composition in fish ponds after liming. Fish ponds limed with dolomitic limestone increased primary production as determined by increased numbers of phytoplankters and greater phytoplankton volumes. The specific composition of phytoplankton communities was not affected obviously by liming. Favorable influences of lime resulted from one or more of the following observed changes in the pond environments: 1) increased carbon for photosynthesis following increases in bicarbonate, 2) greater availability of phosphorus added in fertilizers, and 3) increased concentrations of calcium and magnesium.