Several genetic mutants of fission yeast (Schizosaccharomyces pombe) that form multiple septa and pseudohyphae (i.e., branching growth) have been isolated.(1-15) The current understanding of these mutants is that they lack the ability to separate the two sister cells after formation of the septum. Here it is shown that switching to multisepta and pseudohyphal growth can be induced in a reversible manner in wild-type S. pombe cells by changing the growth conditions, thus indicating an inherent cellular switch. Flow cytometry profiles of exponentially growing cultures of both wild-type and mutant cells further support that a bi-stable switch is controlling the morphological state of the cell in a stress-dependent manner.