A complete chitin synthase gene and one chitin synthase gene fragment of the zygomycete Mucor circinelloides have been cloned and analyzed. Both genes encode zymogenic Class TI chitin synthases. Hybridization analysis showed that there must exist at least another Class II chitin synthase gene in M. circinelloides highly homologous to the cloned Mcchs1 and Mcchs2. The expression of these genes during the dimorphic growing stages was analyzed. Northern hybridizations showed that Mcchs1 transcript accumulates only during the exponentially growing hyphal stage, while no expression could be detected in the yeast form. Expression of Mcchs2 could not be detected at any stage. Accumulation of Mcchs1 transcript was not influenced by visible light. The existence of a multigene chitin synthase family and the observation that Mcchs1 transcription depends upon the dimorphic stage indicate that various chitin synthase activities may have different roles in the dimorphic growth of M. circinelloides.