In order to gain information about the potential interest of the Schizosaccharomyces pombe srb 1 fragile mutants as a host for heterologous protein production, the extracellular secretion of homologous and heterologous invertases was investigated. Under catabolic derepression the fragile srb 1 mutants released into the extracellular medium 5-6-fold more invertase than the parental strain. When transformed with the SUC2 gene, which codes for Saccharomyces cerevisiae invertase, the srb 1-3 fragile mutant, grown under catabolic repression, released into the medium 3-fold more invertase than the wild-type transformant, even though the majority of the enzyme remained associated with the cell wall. Electrophoretic analysis revealed the presence in the fragile strains of some invertase forms with molecular weights smaller than their parallel wild-type strains, suggesting that the srb 1 mutants may underglycosylate not only their homologous but also the heterologous proteins.