Microorganisms have been used intensively by the food industry as well as by pharmaceutical and other chemical industries. A great number of global companies take advantage of fermentation processes in the manufacture of a spectrum of useful products. Many of these processes involve yeasts, which have been used in the production of foods and alcoholic drinks long before the existence of microorganisms was known. Biochemical engineering of microbes has made possible the industrial production of a wide variety of metabolites on a commercial scale at low cost. These metabolites are a tiny proportion of the compounds produced in nature by living organisms. Progress in biological chemistry, genetics, and molecular biology has been put to practical use in the manufacture of foods. Such recent advances have led to the development of a number of commercial products. However, as with other agricultural products, consumers do not necessarily accept so-called engineered or recombinant foods, which involve genetically engineered organisms and enzymes in their production, and the processes themselves may not be generally acceptable because genetic engineering itself may be viewed with doubt. One example is the use of antibiotic resistance marker genes, considered undesirable when incorporated into industrial yeasts because the products are released into the environment. In addition, genetic markers introduced into parent strains for convenience in the selection of mutants may cause practical problems; a new gene once introduced may produce unnecessary metabolites and thereby disrupt the metabolism of the cell, use energy that could be used for growth, or consume a substance that would otherwise be a source of the desired product (1 ). For these reasons, classical genetics and conventional techniques such as mating and protoplast fusion are preferable for use in strain improvement of yeasts. Mating followed by selection is a traditional technique still useful for strain improvement (2,3 ). The use of protoplast fusion to produce hybrid yeasts is a way to solve the problem when a mating reaction does not occur. Protoplast fusion also makes possible the construction of new strains that are hybrids between different species or even different genera.