ABSTRACTBuffer-soluble mycelial proteins of six species ofPythiumwere compared by isoelectric focusing. Species studied includedP. aphanidermatum(P.a.),P. deliense(P.de.),P. myriotylum(P.m.),P. dissotocum(P.di.),P. violae(P.v.),P. ultimumvar.ultimum(P.u.u.) andP.u.var.sporangiferum(P.u.s.).Each species had one or more distinct bands, in addition to many common bands of protein, when focused between pH 4–8. A higher concentration of proteins focused in the acidic region of broad pH range gels. Major protein bands were detected with Coomassie stain. Silver staining increased detection of proteins in low concentration (minor bands) in acidic, neutral, and alkaline regions. Other distinct bands of protein were identified for each species when protein samples were further resolved by focusing in the acidic region (pH 3–6). Several isolates each ofP.a., P.u., andP.de., collected from various regions of North America, produced consistent protein banding patterns with minor variations. Although banding patterns ofP. aphanidermatumdiffered slightly when isolates were cultured on different media, protein patterns were still specifically identifiable. No differences in protein bands were observed betweenP.u.u.andP.u.s.The utility of isoelectric focusing for the differentiation of species ofPythiumis discussed.