The objective of this study was to measure and compare the electrophoretic mobility of a number of microorganisms in aqueous suspensions. The impact of pH, ionic strength and cation valence state on the surface charge properties of the microorganisms was examined. The microorganisms studied include Bacillus subtilis endospores, Cryptosporidium parvum, wild-type Escherichia coli, Escherichia coli 0157:H7, Encephalitozoon intestinalis, and Vibrio cholerae (rough and smooth variants). Includes 13 references, table, figures.