This invention is directed to the transformation of the flavinogenic yeasts, Pichia guilliermondii and Candida famata, and mutants thereof, by electroporation (electrotransformation) and by spheroplast transformation. The invention is also directed to nucleic acid constructs such as vectors, plasmids, and ARS sequences which transform flavinogenic yeasts, and mutants thereof, at a high level and in a stable manner so as to result in stably transformed yeast host cells which express/produce recombinant products. This invention also is directed to flavinogenic yeasts, Pichia guilliermondii and Candida famata, and mutants and temperature sensitive mutants thereof, which produce or overproduce riboflavin.