In vitro tests showed that mycelial growth ofPyrenochaeta terrestris(Hansen) Gorenz, Walker and Larson was inhibited by extracts from noninfected roots and bulbs of the pink-root-resistant onion cultivar Nebuka, and to a lesser extent by extracts from the moderately susceptible and highly susceptible Yellow Sweet Spanish and Southport White Globe, respectively. The active principle was shown to be fungistatic rather than fungicidal. Inhibitory activity of root and bulb extracts from Nebuka was eluted as a single peak from a Bio-Gel P-10 column. The inhibitory compound had anRfvalue of 0.44 on silica gel plates developed in a mixture of methyl ethyl ketone, pyridine, water, and glacial acetic acid (70:15:15:2). The compound reacted positively to ninhydrin, ammoniacal silver nitrate, bisdiazotized benzidine, and diazotized sulfanilic acid. The inhibitory effects of the compound were lost by heating or by exposure to extreme acid or base conditions. The inhibitory compound was without effect when column fractions were diluted 1:30 or more. Quantitative estimation of total phenols in extracts from onion cultivars ranging from highly resistant to completely susceptible showed that resistant cultivars contained about 35% more total phenols than did susceptible ones.