Carrot seed oil is the source of the carotane sesquiterpenes carotol, daucol and beta-caryophyllene. These sesquiterpenic allelochemicals were evaluated against Alternaria alternata isolated from the surface of carrot seeds cultivar Perfekcja, a variety widely distributed in horticultural practise in Poland. Alternaria alternata is one of the most popular phytotoxic fungi infesting the carrot plant. The strongest antifungal activity was observed for the main constituent of carrot seed oil, carotol, which inhibited the radial growth of fungi by 65% at the following concentration.