The kinetics of agglutination and adhesion of Aeromonas salmonicida, the agent of furunculosis, were investigated with six strains. Three pathogenic strains were autoagglutinative and adhesiveto fish tissue culture (RTG-2 cells), whereas the other three nonpathogenic strains lacked these capacities. The capacities for agglutination and adhesion were inhibited by EDTA, although treatment with pronase and lysozyme produced no inhibitory effect on these capacities. With respect to net electrical charges of cells of agglutinating and nonagglutinating strains, a large difference was detected by electrophoresis; cells of autoagglutinating strains carried negative net electrical charges, and cells of nonagglutinating strains showed positive net electrical charges. Spheroplasts, induced from cells of both strain groups by cultures with a hypertonic medium supplemented with a high concentration of penicillin G, preserved the difference in the capacity for agglutination and in the net electrical charges. Thus, the autoagglutination and adhesion are considered to be due to negative net electrical charges in the interior or surface of spheroplasts.