Pseudomonas aeruginosa 142 and a presumed variant were grown axenically in chemostats on salicylate/benzoate or salicylate/glucose binary feeds. Each substrate was supplied at 2, 10, 50, 90, 98, or 100% of the total energy flux. Two experiments were also run with ternary mixtures using the same substrates. Aliquots were transferred to fed-batch reactors receiving the same substrates at the same specific rates as the chemostat, but with one substrate radiolabeled with 14 C. Radiolabel incorporated into biomass, 14 CO 2 , and soluble microbial products over a period of 8 minutes was used to establish the biomass yield, CO 2 yield, and product yield, respectively, associated with a given substrate. The effect of the percent substrate in the feed on the yields depended on the pair of substrates supplied. When benzoate comprised 50% or more of the applied substrate in salicylate/benzoate feeds, the fraction of benzoate in the feed had a small effect on the yield values associated with benzoate. However, when benzoate constituted 2% or 10% of the feed, CO 2 yields were lower, biomass yields were slightly lower, and product yields were higher. In contrast, the percent of salicylate in the feed had little effect on any of the salicylate yields for cells growing on the salicylate/benzoate feeds. When salicylate was mixed with glucose, the yields associated with salicylate behaved quite differently. Biomass and CO 2 yields were lower and product yields higher when salicylate was 2% or 10% of the feed than when it was higher. In the same substrate mixtures, glucose-based biomass yields were higher and CO 2 yields were lower when glucose constituted 2% or 10% of the feed but were constant for higher percentages. The results suggest that the fate of a substrate is relatively independent of the feed composition as long as the substrate in question constitutes a significant percentage of the mixture. Thus, in those situations the assumption of a constant biomass yield in multicomponent substrate modeling is justified. However, when a given substrate constitutes a small percentage of the feed, significant changes in yield may occur.