Saccharomyces cerevisiae FUS3/DAC2 protein kinase, a homolog of mammalian mitogen-activated protein (MAP) kinase, inactivates a G1 cyclin encoded by the CLN3 gene to arrest cell division in the G1 phase and activates a transcriptional factor STE12 in response to mating pheromone during sexual conjugation. To elucidate the role of the FUS3/DAC2 gene product in the mating process, I constructed and characterized dac2 cln3 double mutants. Here, I show that FUS3/DAC2 is required for completion of cell fusion even in the dac2 cln3 double mutants in which the pheromone response is restored, suggesting that FUS3/DAC2 plays a positive role in cell fusion during conjugation. In addition, the cdc dac2 and cdc37 ste double mutants were constructed and investigated for their phenotypes to clarify the relationship between FUS3/DAC2, STE7 or STE11 and CDC gene products (CDC28, 36, 37 and 39). The results indicate that FUS3/DAC2 may act upstream of CDC28 and provide evidence that the G1 arrest and morphological changes conferred by the cdc37 mutation may require FUS3/DAC2 (MAP kinase), STE7(MEK) and STE11 (MEK kinase).