The mitogen-activated protein kinase Hog1 gene (Kmhog1) was isolated from Kluyveromyces marxianus strain NBRC 1777 by degenerate PCR and genome walking, and then disrupted to construct a mutant strain hog1a dagger. The mutant was now more sensitive to acetic acid and its growth was nearly completely inhibited by 0.5 M NaCl (97%) and 10 mM H2O2 (93%) as compared with the wild-type cells. However, neither strain grew at 47A degrees C. Kmhog1 may thus be required for adaptation to acetic acid, osmotic, and oxidative stress but is not involved in thermotolerance.