The sequence and characterization of the KlGSK-3 gene in chromosome VI [corrected] from Kluyveromyces lactis is presented. The deduced amino-acid sequence predicts a protein of 415 amino acids and an Mr of 47 kDa. A computer search reveals significant homology to serine/threonine protein kinases closely related to members of the GSK-3 subfamily. The Klgsk-3::URA3 disrupted strain is unable to grow in glucose at 37 C but KlGSK-3 is not essential for vegetative growth at 30 C or 14 C. The KlGSK-3 gene presents the highest homology with the Saccharomyces cerevisiae MDS1 gene. Expression studies show an increase of mRNA levels caused both by carbon starvation and when diploids are shifted from rich to sporulation media. The data reported show that KlGSK-3, like MCK1 from S. cerevisiae, is related to glycogen storage.