Dafadine-A, an analog of dafadine, is a novel inhibitor of DAF-9 cytochrome P450 in the nematode Caenorhabditis elegans; also inhibits the mammalian ortholog of DAF-9(CYP27A1). IC50 value: Target: DAF-9(CYP27A1) inhibitor The DAF-9 cytochrome P450 is a key regulator of dauer formation, developmental timing and longevity in the nematodeCaenorhabditis elegans. Here we describe the first identified chemical inhibitor of DAF-9 and the first reported small-molecule tool that robustly induces dauer formation in typical culture conditions. This molecule (called dafadine) also inhibits the mammalian ortholog of DAF-9(CYP27A1), suggesting that dafadine can be used to interrogate developmental control and longevity in other animals.