The Bureau of Mines has investigated the surface charge of amphibole asbestos fibers and nonasbestiform cleavage fragments, as part of the Bureau's fundamental studies to provide new knowledge on the physical and chemical properties of the amphibole minerals. Microelectrophoresis was used to measure the electrophoretic mobilities, in water, of particles from six asbestiform amphiboles and seven nonasbestiform amphiboles over the ph range of 5 through 9. The particles generally ranged in size from 2 to 5 um wide by 10 to 30 um long. The iso-electric point was determined for four of the minerals, and values were found to be between ph 3.5 and ph 5. The minerals studied had zeta potentials that generally ranged from - 25 to -50 mv in neutral solution. Attachment of colloidal positively charged agi was also studied as a means of determining surface charge. Scanning electron microscopy of amphibole particles that had been in contact with aqueous agi showed equal attachment of agi to asbestos fibers and to cleavage fragments; the extent of agi coating was the same for both. The electrophoretic mobilities of asbestos fibers were essentially the same as those of elongated cleavage fragments of the same aspect ratio (particle length divided by width). Blocky particles from both asbestiform and nonasbestiform specimens had lower mobilities.