Two plate count methods are proposed for direct assessment of the maximum specific growth rate (microm) of bacteria in mixed communities. An estimate of microm of individual colonies is obtained by plating samples on an agar medium and determining either the time required to form macroscopically visible colonies, Tv (diameter < or =0.2 mm), or the linear radial growth rate, Kr, of single colonies. In accordance with theoretical models, a linear relationship was found between microm determined in liquid culture and 1/Tv, and between microm and Kr. Empirical relationships were established for these relationships. The time required to form a visible colony was 17 +/- 9 h longer for cells in the stationary growth phase, whereas the linear radial growth rate was not affected by the physiological state of the cells. The proposed plate count methods are simple and applicable for describing the community structure, and for estimating the frequency distribution of maximum specific growth rates in mixed communities. By using this frequency distribution it is possible to calculate diversity indexes and to assign a microbial community a position on an r/K-gradient.