Acquired resistance to the antibiotic cycloheximide developed byNeurospora crassa andSclerotium rolfsii was studied. Both the fungi gained certain level of tolerance to the antibiotic just after a single exposure and by serial transfers could adapt to the several-fold initial inhibitory dosage. Cycloheximide-resistance in both the cases was not a stable characteristic and was lost gradually on serial transfers in fungicide-free media. The resistant strains of both the fungi showed poor growth and decreased sporulation or sclerotia formation in fungicide-free media.Sclerotium rolfsii developed cross-resistance to Difolatan but not to Polyoxin-D, Hinosan and Bayleton. The cycloheximide-resistance inSclerotium rolfsii was probably due to the conversion of cycloheximide into isocycloheximide which is a less toxic structural analog, as revealed by thin layer chromatographic studies of culture filtrates of resistant and sensitive strains. The resistant strain ofSclerotium rolfsii retained its pathogenicity to tomato, mustard and chilli seedlings. However, the loss of sclerotia forming capacity and the instability of the acquired resistance trait may prove to be of ecological disadvantage to the resistant strain
