The surface charge of different isolates of Phytomonas from Euphorbia hyssopifolia, Euphorbia pinea, Euphorbia characias and Manihot esculenta was analysed by the binding of cationic particles (colloidal iron hydroxide at pH 1.8 and cationized ferritin at pH 7.2) to the protozoan surface and by determination of the cell electrophoretic mobility (EPM). All the isolates had a net negative surface charge, and the isolate from E. hyssopifolia manifested the greatest negative charge. A good relationship between the electrophoretic mobility data and the density of the cationic particles on the parasite's surface, as seen in ultrathin sections, was observed. Neuraminidase treatment did not significantly reduce the mean EPM of the parasites analysed.