Intergeneric hybrids of an Arg auxotroph of Kluyveromyces marxianus able to grow up to 52 degrees C and Saccharomyces cerevisiae capable of growth up to 40 degrees C, have been constructed by protoplasmic fusion. The fusants resembled the former except that they were prototrophic and some produced ethanol in excess of 6% (v/v) both at 30 and 45 degrees C, an attribute otherwise lacking in the parents. The hybrids were stable during mitotic and meiotic growth. Genetic evidence leading to the retrieval of orthodox arg at a frequency of 6% from the prototrophic hybrids, confirmed their genuineness and the presence of S. cerevisiae-specific ARG(+) in an integrative manner. The integration seemed to have occurred at a locus similar to 12 cM away from the orthodox arg which on the other side was found to be linked to MET. The order of three genes was, therefore, speculated to be either MET-arg-ARG(+) or ARG(+)-arg-MET.