Four strains ofAzospirillum were ranked according to numbers of cells adsorbed on the roots of seedlings in liquid medium, and the rankings were evaluated for their usefulness in predicting success of colonization of the roots of pot-grown plants.Different rankings were observed on different parts of the roots and on different host plants. Rhizosphere colonization results for rice were similar to those for clover and showed little difference between bacterial strains. The population densities were approximately equal to those of the most dense strains in the wheat rhizosphere, whereas the highest concentrations in the root interior of clover and rice were only about one-tenth of those in wheat.Rankings of initial adsorptive ability on various parts of the roots showed potential for predicting the best strains for colonizing the root interiors of wheat and clover. On wheat, the two strains (Cd and SpBr14) which showed best initial adsorption to the root cap were best at colonizing the endorhizosphere of pot-grown plants. For rice, strains Cd and SpBr14 gave lowest and highest values, respectively, both for adsorption to the terminal 2 cm of roots and for subsequent colonization of the root interior. Data on initial adsorption were of no value in predicting the relative success of strains in colonizing the root surface of any host plants or the interior of clover roots.