Antifungal activity in diffusate and tissues of stored carrot roots was studied. Three main compounds inhibitory to Mycocentrospora acerina were separated. Of these, 6-methoxymellein and p-hydroxybenzoic acid were identified with the aid of chromogenic agents, u. v. spectroscopy and low resolution mass spectrometry. A third inhibitor, which has subsequently been identified as falcarindiol, was also found. ED50 values for inhibition of chlamydospore germination were 79 μg ml−1 for 6-methoxymellein and 292 μg ml−1 for p-hydroxybenzoic acid. 6-Methoxymellein accumulated to levels above the ED50 in diffusate from roots and in the locality of restricted lesions. The possible role of these compounds in carrot root resistance to Mycocentrospora acerina is discussed.