Pseudohyphal growth in Saccharomyces cerevisiae has been postulated to be an adaptation to foraging for nitrogen during nitrogen starvation. This process was described as a strictly diploid phenomenon which did not occur in haploid yeast cells and was under the genetic control of both the mating-type locus and a group of five genes, the BUD genes, regulating bud formation. We have also observed a dimorphic growth pattern in yeast growing on various nitrogen-limiting synthetic media. However, and in contrast to a previous report, we find that pseudohyphal growth is not precluded in haploid cells. We demonstrate that haploid pseudohyphal growth is strictly oxygen-dependent and is rapidly reversible, defining pseudohyphal growth as a reversible developmental pathway in yeast.