The methylotrophic yeast, Candida boidinii, was investigated as a new efficient host for heterologous gene expression. The Saccharomyces cerevisiae adenylate kinase gene (ADK1) was used as the first example for heterologous enzyme production in C. boidinii. C. boidinii cells were transformed with plasmids harboring the S. cerevisiae ADK1 gene under the alcohol oxidase (C. boidinii AOD1) promoter, The chromosome-integrant strains produced adenylate kinase protein corresponding to 22%-28% of the total soluble proteins in an enzymatically active form. When the three-copy integrative transformant was grown for 60 h on methanol-glycerol medium in a 1.5-1 jar fermenter, adenylate kinase was produced intracellularly with a yield of up to 2 g/l culture medium. As the expression of the S. cerevisiae ADK1 in C. boininii was under similar regulation to that of the C. boidinii AOD1, the previously cloned 1.7-kb AOD1 promoter fragment was proved to harbor sufficient cis elements for AOD1 regulation and found to be an efficient promoter for heterologous gene expression.