SUMMARYThe reversion of the highly sphericalras1−phenotype of the sterile mutant ofSchizosaccharomyces pombewas examined by fluorescence microscopy of Calcofluor‐ and rhodamine phalloidin‐labelled cells and by freeze‐fracturing. Rapid and dramatic changes occurred in cell shape after a nutritional stimulus, with cells regaining their rod‐like cell shape during successive cell divisions. This was achieved by restoration of the cell wall architecture and of the invaginated state of the plasma membrane. This is the first demonstration of a nutrient‐induced reversion ofras1−S. pombecells from rounded to rod‐