Dimorphism in K_. fraqilis is a strain dependent phenomenon. ~Concdrtians have been established whereby Kluvyeromycss fraqi lis WRRL y 2415 can be induced to grow either in a yeast-like (Y) or filame ntous ( F ) mode of growth. Oxygen av/ai lability is a determining factor which induces the dimorphic change in batch culture conditions. Anaerobic culture conditions which favour fermentation also promote yeast-like morphology, whereas oxidative conditions promote filamentous development. Increased cellular cytochrome content and oxygen uptake rates accompany the Y-F transition which suggest that mitochondrial functions are needed for the morphological transformation. Additional evidence in support of mitochondrial control of morphogenesis in K_. fraqilis IMRRL y 2415 came from studies using chloramphenicol. This inhibitor prevented filamentation and induced the formation of respiratory-deficient budding cells in semiaerobic cultures. When K_. fraqilis MRRL y 2415 was grown aerobically under lactose limited conditions, morphology was related to growth rate. A transition from round to cylindrical shaped cells occurred during a shift from low growth rates (□ = 0.1 lir” ^) to higher specific growth rates (□ = 0.4 hr- ^). It appeared that nutrient limitation had an influence on the morphogenetic expression of j<. fraailis 'iRRL y 2415. The filamentous form of K_. fraqi lis NRRL y 2415 was characterised by a more rapid lactose utilisation and ethanol production rate than the yeast form although maximum ethanol concentrations were not significantly different. Yield coefficients for both ethanol and biomass production were higher for anaerobic (yeast phase) cultures than for aerobic (filamentous) cultures.