In vitro screening of some essential oils against Fusarium oxysporum Schlecht and F. udum Butler which cause wilt diseases of pigeonpea (Cajanus cajan (L.) Millsp.] revealed that the oil of Trachyspermum ammi (ajowain) was a strong fungitoxicant. The toxicity of the oil was found to be fungicidal at 0.1 %, which inhibited heavy doses of inocula (25 fungal discs, each of 5-mm diameter) and killed the test pathogen in just 2-3 s. The oil's activity was found to be thermostable up to 80°C and autoclavable at 15 pounds per square inch pressure. The oil exhibited broad antifungal activity against Alternaria alternata (FR.) Keissler, Aspergillus flavus Link, A. parasiticus Speare, Cladosporium cladosporioides (Fresenius) de vries, Colletotrichum capsici (Syd.) Butler & Bisby, C. falcatum Went, Curvularia lunata (Wakker) Boedijn, Helminthosporium maydis Nisikado & Miyakel, H. oryzae Breda de Haan, Penicillium implicatum Biourge, P. italicum Wehmer, and P. minio-luteum Dierckx, and was found to be more active than some commercial synthetic pesticides (benlate, captan, mancozeb, and thiram). The oil did not have any phytotoxic effects on seed germination and seedling growth of the host plant, C. cajan. The oil's toxicity prevents the wilt disease of pigeonpea at 10% concentration. It has also been reported to be harmless to human health.