Comparative investigations on the substrate utilization of Nocardia amarae and Rhodococcus spp. (R. rhodochrous and R. erythropolis) were carried out using various fatty acids and paraffinic hydrocarbons. Upon calculation and comparison of their specific growth rates (mu) during the logarithmic phase, it was shown that the lower fatty acids (C2-C5) were utilized preferably and the highest mu value was obtained in octadecane (C18) for all three strains. The initial total organic carbon (TOC) concentration affecting the growth of the 3 strains was investigated, using octadecane as the carbon source. From a comparison of the kinetic parameters of the 3 strains with those obtained in the past studies, it was found that Rhodococcus spp. had higher growth rate and lower affinity for octadecane than N. amarae.