The addition of compounds able to peroxidize cell lipids (carbon tetrachloride (CCl4). cumene hydroperoxide (CUH), or linoleic acid hydroperoxide (LAH)) to 5‐day‐old Czapek‐Dox Medium cultures ofAspergillus parasiticusinduces a significant reduction of the tri‐unsaturated ergosterol (ERG) levels in fungal microsomes and mitochondria, whereas the concentrations of the di‐unsaturated linoleic acid (LA; C18:2 n‐6) are unaffected. Aflatoxin (AFT) output follows ERG reduction and is associated with both a renewal of fungal growth and a slow increase of ERG concentration in subcellular membranes. We suggest that, by analogy with the regulatory role played on cell proliferation and metabolism by polyunsaturated fatty acid by‐products (eicosanoids) in mammalian membranes, by‐products of ERG oxidation may be considered triggers sufficient to induce both further fungal growth and AFT biosynthesis.