Studies of metabolic regulation in yeasts have a long history. Yeasts have served as models for the regulation of fermentaive metabolism. Yeasts differ in their partitioning of metabolism between respiration and fermentation, that is, between their use of oxygen and organic compounds as terminal electron acceptors. The respirative route is assumed to yield more ATP. Many yeasts carry out metabolism solely by the respirative route. Other show a predominantly fermentaive metabolism, even when oxygen is freely available. Fermentaion under aeobic conditions, that is, when respiration should be possible, was believed to result from repression of respiration by fermentation. This phenomenon is known as the Crabtree effect. However, recent work has shown that aerobic fermentation results from an inherently-limited respiratory capacity of some yeasts, rather than from a specific repression of respiration, Nevertheless, considerable enzymatic evidence exists that suggests that a Crabtree effect does indeed operate in some yeasts, specifically Saccharomyces cerevisiae. Because multiple electron transport systems are known to exist in yeasts, repression of the normal ATP-producing system can be accompanied by the induction of an alternate pathway. No decrease in the overall rate of oxygen utilization would then be apparent. Repression would, however, affect the yield of ATP from oxidative metabolism. This effect should be detectable using a suitable analysis of growth energetics. To this end, a model has been developed and applied to a variety of yeasts in order to examine them for changes in respiratory efficiency indicative of a Crabtree effect. A Crabtree effect consistent with previous enzmatic findings was detected in S cerevisiae and S. uvarum, but not in Schizosaccharomyces prombe. New regulatory classifications based on model findings are proposed and methods for independently verifying these findings are outlined.