Sporulation, spore germination, and yeast-hypha dimorphism in the filamentous fungus Mucor racemosus provide useful model systems to study cell development in eucaryotic cells. Three RAS genes (MRAS1, MRAS2, and MRAS3) from M. racemosus have been cloned, and their nucleotide sequences have been determined. The predicted amino acid sequences and the sizes of the three MRAS proteins exhibit a high degree of similarity with other ras proteins, including that encoded by H-ras, which have been implicated in regulation of proliferation and development in eucaryotic cells by mediating signal transduction pathways. The MRAS proteins show conservation of functional domains proposed for ras proteins, including guanine nucleotide interaction domains, an effector domain, a binding epitope for neutralizing antibody Y13-259, and the COOH-terminal CAAX box, which is a site of thiocylation and membrane attachment. Amino acid sequences unique to each MRAS protein occur adjacent to the CAAX box, consistent with the location of the hypervariable region in other ras proteins. Northern (RNA) analysis was used to study expression of the three MRAS genes in relation to cell development. Gene-specific probes for two of these genes, MRAS1 and MRAS3, hybridized to different 1.3-kb mRNA transcripts. The accumulation of these transcripts depended on the developmental stage, and this pattern was different between the two MRAS genes. No transcript for MRAS2 was detected in the developmental stages examined. The unique patterns of MRAS transcript accumulation suggest that individual MRAS genes and proteins may play distinct roles in cell growth or development.