A foam-forming and a floc-forming species were grown on two specific media, Czapek- Yeast and Czapek-Hexadecane, to determine their kinetic constants, mu(max) and K(S). A comparison of these data, together with data reported previously for filamentous and flocculent species, showed that there appeared to be some anomalous behaviour when previously established criteria were used to judge whether species dominance would occur. Possible reasons for these anomalies are discussed