Field-flow fractionation (FFF) techniques were used to separate various strains of bacteria and differentiate live from dead bacteria. The sedimentation, flow, and electrical FFF separations were accomplished in less than 15 minutes using the rapid by perlayer mode. The bacteria used in these studies include Pseudomonas putida, Escherichia coli, and Staphylococcus epidermidis. Sedimentation FFF gives the highest resolution separations of a mixture of spherical and rod-shaped bacteria, and a mixture of two rod-shaped bacteria.