The Bureau of Mines, in cooperation with the Florida phosphate mining industry, investigated the dewatering of waste phosphatic slimes. Part of this investigation was to determine the electrophoretic mobilities and cation exchange capacities of the slimes. The mobilities and exchange capacities are reported for samples of slimes taken from 15 plants operating in the Florida Land- Pebble Phosphate District that includes Polk, Hillsborough, Manatee, Hardee, and DeSoto Counties. The electrophoretic mobilities determined were all negative, ranging from -0.24 To -1.93 (Micrometers per second) per (volt per centimeter). Also reported are frequency distributions of electrophoretic mobilities, electrophoretic mobilities of slime diluted with distilled water, and electrophoretic mobilities of sand-slime mixtures. The cation exchange capacities ranged from 17 to 45 milliequivalents per 100 grams of slime solids. Work done in cooperation with the University of Alabama.