sep1(+), the Schizosaccharomyces pombe homologue of the forkhead/HNF-3 transcription factors, plays a role in the cell separation at the end of mitosis. Its inactivation by interruption of the coding region is not lethal but renders the sister cells unable to separate and causes hyphal growth. The persistence of unsplit septa indirectly interferes with the establishment of new cell polarity by preventing cell growth at cell tips. Temporal changes in the transcription of sep1(+) correlate with the cell cycle progression showing maximal expression at the peak of cell plate index in synchronized cultures. The constitutive overexpression of sep1(+) has no discernible effect on the morphology and physiology of the cells.