Aspergillus nidulans is a non-pathogenic fungus with well-developed genetics which provides an excellent model system for studying different aspects of drug resistance in filamentous fungi. As a preliminary step to characterizing genes that confer pleiotropic drug resistance in Aspergillus, we isolated cycloheximide-sensitive mutants of A. nidulans, which is normally resistant to this drug. The rationale for this approach is to identify genes whose products are important for drug resistance by analysing mutations that alter the resistance/sensitivity status of the cell. Fifteen cycloheximide-sensitive (named scy for sensitive to cycloheximide) mutants of A. nidulans were isolated and genetically characterised. Each scy mutant was crossed with the wild-type strain and five of the crosses gave 50% cycloheximide-sensitive progeny suggesting that they carry a single mutation required for cycloheximide sensitivity. We examined ten scy mutants for resistance/sensitivity to other drugs or stress agents with different and/or the same mechanism of action. Six of these mutants exhibited other altered resistance/sensitivity phenotypes which were linked to the cycloheximide sensitivity. These six mutants were analyzed by pairwise crosses and found to represent six linkage groups, named scyA-F. One of the mutants showed fragmentation of its vacuolar system and, in addition, its growth was osmotic, low-pH, and oxidative-stress sensitive.