Two strains of "Microthrix parvicella" (RN1 and 4B strains) have been physiologically and kinetically characterized by utilizing a new technique particularly suitable for slow growing bacteria. The proposed method is based on the Total Extended Filament Length (TEFL) measure on filaments grown on agar surfaces. This allows us to study more easily and rapidly slow growing bacteria under many different conditions without altering the growth environment during the test. "Microthrix parvicella" appears to have versatile physiological capabilities allowing effective competition against other bacteria in the complex biocenosis of activated sludge. Main peculiarities of this organism are its storage capabilities under an aerobic/anoxic/anaerobic reaction environment, the resistance to long periods of anoxic/anaerobic conditions, the possibility of growing with appreciable kinetics at very low temperatures. In the kinetic characterization low Ks values have been observed; this finding supports the assumption that "Microthrix parvicella" is a "Ks- strategist" with high substrate affinity. Kinetics tests performed in a mixed population of activated sludge bacteria and on pure culture of filaments isolated from the same sludge sample showed that no beneficial effect on the growth rates of "Microthrix parvicella" seems attributable to the presence of activated sludge. This last result confirms the validity of pure culture studies for this microorganism and the reliability of physiological and kinetic data so evaluated in describing real systems.