The amount of growth and external morphology of Candida albicans growing in a defined medium was influenced by the amount of yeast extract supplemented to the medium. With 10mg/ml of yeast extract a maximum growth exclusively consisting of yeast-like (Y) morphology was obtained, while cells grown in a medium containing a suboptimal concentration of yeast extract for growth (1mg/ml) predominantly developed into filamentous (F) form. Comparative study of macromolecular content of both Y and F phase cultures revealed that RNA and protein levels in the former culture were 5- and 2-times higher, respectively, than those of the latter, although values for both cultures fluctuated to some extent during 24hr of incubation. DNA content was nearly the same in both phases. When stationary phase Y cells were placed in the yeast extract-optimal fresh medium, transient repression of respiration induced by glucose in the medium was observed as is known to be characteristic of facultative microorganisms. In contrast, the respiratory pattern of F cells transferred into the yeast extract-suboptimal medium was significantly different from that of Y cells and rather comparable with that of obligately aerobic yeasts like Rhodotorula sp. Such a correlation of macromolecular composition and respiratory pattern with developing cell morphology was ascertained by experiments, in which Y to F and F to Y phase conversion were induced by transferring initial Y phase and F phase cells into yeast extract-suboptimal and -optimal media, respectively.