1. Auxotrophic mutants of Trichoderma harzianum tolerant to carbendazim were obtained by UV-light mutagenesis and carbendazim induction on PDA medium.

2. The isolation of these mutants would provide a useful auxotrophic marker to facilitate the development of an efficient host-vector system in K. lactis.

3. Prototrophic hybrids have been obtained by the protoplast fusion between auxotrophic haploid and diploid mutants of Saccharomyces cerevisiae.

4. Four possible genotypes that were prototrophs, auxotroph of one parental type, auxotroph of the other parental type, and auxotrophic recombinants can be generated from progenies of somatic hybrids.

5. Further investigation demonstrated that UV light decreased the mating ability of ade auxotrophic haploids and had no significant effect on the mating ability of other auxotrophic haploids.

6. The optimal conditions for protoplast isolation and regeneration of 4 auxotrophic mutant strains of Trichoderma were studied.