A mutant of Kluyveromyces lactis is described which did not grow with substrates giving the Kluyver effect. In addition it could not grow with non-fermentable carbon sources, although it was not respiratory deficient. Abolition of respiration by cyanide also caused inability to grow with substrates showing the Kluyver effect in the wild-type strain. When the yeasts were using substrates showing the Kluyver effect, shifting to anaerobic conditions gave an immediate decrease in the intracellular concentration of d-glucose 6-phosphate. The results obtained were consistent with the need of a common respiratory and/or anabolic pathway for the utilization of these substrates.