Agrobacterium tumefaciens B6 and ATCC 15955 were grown under octopine or glutamate limitation in chemostats. Examination of the maximum specific growth rate (mu max) and substrate affinity (KS) for each strain indicated that strain B6 was highly inefficient in its use of octopine as either a nitrogen or carbon source compared with strain ATCC 15955. Examination of the yield coefficients showed that in both strains octopine was used more efficiently as a nitrogen source than as a carbon source. The data permitted predictions to be made concerning the outcome of competition for a single limiting substrate. Under octopine limitation, strain ATCC 15955 should dominate; under glutamate limitation, strain B6 should dominate. The results of an observed competition with glutamate as the limiting substrate confirmed the latter prediction, although B6 did dominate at a rate faster than was predicted from simple competition theory. B6 displayed higher growth rates and substrate affinities than ATCC 15955 on all concentrations of glutamate. The yield of B6 on octopine was also considerably higher. This latter attribute could provide an ecological advantage to B6 because of the importance of yield in the fate of competitions under multisubstrate regimens. These will be the most prevalent regimens in the area around the tumor (tumorosphere) or the rhizosphere. The increased performance on glutamate could provide an advantage in an opine-free environment when B6 is growing as a saprophyte.