Breaking of hyphae derived from growth of the phytopathogenic fungus Crinipellis perniciosa in liquid media yielded cell aggregates that performed as "quasi single cell" in toxicity assays. When treated with the chemical mutagens 4-nitroquinoleine-1-oxide (4NQO), hydrogen peroxide (H2O2), or paraquat (PAQ) as well as with ultraviolet light (UVC), broken hyphae of C. perniciosa gave a single cell-like response, i.e., survival curves similar to those obtained when treating single-cell suspensions. C. perniciosa had significantly higher UVC resistance than haploid bakers yeast but was more sensitive to 4NQO and extremely sensitive to PAQ and H2O2 when compared to likewise-treated yeast. Haploid C. perniciosa basidiospores (monokaryotic) were significantly more UVC resistant than C. perniciosa broken hyphae and than haploid and diploid yeast wild-type strains. This suggests a high capacity in C. perniciosa for repair of UVC-induced DNA lesions or, alternatively, an efficient protection from UVC irradiation, especially in basidiospores. The pronounced sensitivity of the dikaryotic form of C. perniciosa to PAQ and H2O2 points to a weak protection from oxidative stress-inducing agents.