The genetic analysis and characterization of protoplast fusion hybrids between complementary auxotrophic mutants of Candida pseudotropicalis were carried out. Nuclear fusion appeared to occur in fusion hybrids (e.g., F15 and F33), as strongly suggested by isolation of recombinants after mitotic segregation of parental genetic markers. This was confirmed by KNA content, nuclear staining and comparison of survival rate to UV light. After keeping fusion hybrids for approximately one year, the frequency of spontaneous mitotic segregation was $3.0 imes 10^{-4}$ - $8.1 imes 10^{-4}$ while that of induced mitotic segregation was $1.4 imes 10^{-3}$- $1.7 imes 10^{-3}$. These results suggested that they maintained stable karyogamy state. It was also found that the production of $eta$-D-galactosidase from F15, F33 and F158 was somewhat increased when compared with that from either auxotrophic parents or wild type.