A mutagenic compound, designated aquilide A, was isolated and identified from bracken fern (Pteridium aquilinum (L.) Kuhn). Aquilide A was found to be responsible for greater than 50% of the mutagenic activity observed after incubation of the methanol extract at alkaline conditions. This compound was found to be a potent genotoxic compound in mammalian cells in vitro and therefore, may be responsible for at least part of the carcinogenic activity of bracken. The instability of aquilide A under conditions normally encountered during analytical chemical procedures may explain why so many efforts to elucidate the carcinogenic factor(s) present in bracken have been unsuccessful.