Fission yeast cells grow by extension at the ends (poles) and divide by transverse fission. It has previously been reported that Schizosaccharomyces japonicus var. japonicus can switch to unipolar, filamentous growth. Here it is shown that the yeast-to-mycelium transition is a gradual process involving a changeover to unipolar growth associated with asymmetric divisions, the development of large polarly located vacuoles, the modifications of the actin and microtubular cytoskeleton and the repression of cell separation after division. High concentrations of glucose in the medium or supplementation of the medium with caffeine or cAMP support the bipolar yeast phase, inhibit the transition to the mycelial phase and induce the conversion of hyphae to yeasts. These effects suggest that cAMP may be involved in the regulation of dimorphism. Temperatures below 18 degrees C or over 35 degrees C are restrictive for the mycelial phase and provoke a return to yeast phase.