SUMMARYBased on their immunological and biochemical properties, physiological relationships among the three available species ofPhymatotrichum, P. fimicola, P. fungicolaandP. omnivorumwere investigated. Pyrolysis-gas-liquid chromatographic (PGLC) analyses of freeze-dried organisms showed very little or no difference between the“fingerprints”ofP. fimicolaandP. fungicola.However, a distinct“fingerprint”was observed fromP. omnivorum.The fatty-acid profile ofP. omnivorumwas also distinct, as in PGLC, from the other two species. Serological studies revealed that antisera toP. fimicolaandP. fungicolagave positive reactions to all three antigens but antisera toP. omnivorumgave positive reaction only withP. omnivorumantigen. This study confirms thatP. omnivorumis different both physiologically and immunologically from the other two species in addition to the differences in their morphological characters. On the other hand,P. fimicolaandP. fungicolawere found to be related both immunologically and physiologically although they differ in morphological characters.