The inherent pleomorphism of Bacterionema matruchotii resulting from its mode of reproduction was enhanced by temporal effects of culture age and growth condition and by the more lasting effects of rough to intermediate to smooth morphological dissociation. Routine morphological observations with a single growth condition were inadequate to permit unambiguous judgements of culture purity. Multiple criteria were required. Pleomorphism and the undetected presence of contaminants in primary and successive cultures of B. matruchotii can explain the emergence of unrelated bacteria from B. matruchotii reported previously by others and ascribed to genetic instability.