We have identified a Schizosaccharomyces pombe gene, mkh1, that encodes a MEK kinase (MEKK) homolog. The coding region of mkh1 is contained within a single exon encoding a 1,116-amino-acid protein. The putative catalytic domain of Mkh1 is 54% identical to the catalytic domain of S. cerevisiae Bck1, the most closely related protein. Deletion of mkh1 did not significantly affect cell growth or division under standard conditions. However, mkh1delta cell growth was inhibited by high KCl or NaCl concentrations. mkh1delta cells required a longer time to reenter the cell cycle after prolonged stationary-phase arrest. Also, mkh1delta cells exhibited a round cell shape, while overexpression of Mkh1 resulted in an elongated cell shape. mkh1delta cells exhibited a more dramatic phenotype when grown in nutrient-limiting conditions at high temperature or in hyperosmotic medium. In such conditions, completion of cytokinesis was inhibited, resulting in the growth of pseudohyphal filaments with multiple septa and nuclei. Also, mkh1delta cells were hypersensitive to beta-glucanase treatment. Together these results suggest that Mkh1 regulates cell morphology, cell wall integrity, salt resistance, cell cycle reentry from stationary-phase arrest, and filamentous growth in response to stress. These phenotypes are essentially identical to those exhibited by cells lacking Pmk1/Spm1, a recently identified mitogen-activated protein kinase. Our evidence suggests that Pmk1/Spm1 acts downstream from Mkh1 in a common pathway. Our results also suggest that Mkh1 and Pck2 act independently to maintain cell wall integrity, cell morphology, and salt resistance but act in opposition to regulate filamentous growth.