In the course of our research project on plant growth regulators among fungal metabolites we noticed that an Aspergillus terreus strain, C-520, which was isolated from a soil sample collected in Takarazuka city, produced several plant growth regulators. One of them is a plant growth promoter and its structure elucidation is in progress. Three of them are plant growth inhibitors. In this paper we wish to report the isolation, identification and biological activities of these three plant growth inhibitors produced by Aspergillus terreus C-520. Asp. terreus C-520 was cultured in Czapek-Dox medium (40 liters) fortified with yeast extract on a shaking machine for three days at 30°C. The culture filtrate was extracted with ethyl acetate and the neutral extract was concentrated in vacuo. The concentrate was subjected to silicagel column chromatography (Wako gel C-200). Elution was performed successively with chloroform and chloroform-acetone, increasing stepwisely the content of the latter. The eluate with chloroform and that with chloroform-acetone (80: 20) showed inhibition of growth of lettuce seedlings (Lactuca sativa L., Great Lakes 366). A growth promoter for lettuce seedlings was contained in the eluate with chloroform-acetone (90: 10). The eluate with chloroform was further chromatographed on a silicagel column with a solvent system of n-hexane and ethyl acetate (80: 20), and then a Sephadex LH-20 column with methanol. Two biologically active substances designated as C-520A (I) and C-520B (II) were isolated as yellow crystals (30 mg) and a yellow powder (5 mg), respectively. The eluate with chloroform-acetone (80: 20) was rechromatographed on a silicagel column with a solvent system of n-hexane-ethyl acetate. Crude crystals of an active substance were obtained from the eluate with n-hexane-ethyl acetate (60 : 40). Recrystallization from acetone-ethyl acetate gave colorless needles of C-520D (III) (200 mg). I was deduced to be bisdethiodi(methylthio)-acetylaranotin, II to be acetylaranotin, and III to be terrein based on elemental analysis, IR, high resolution mass spectrum and ¹H-NMR data. The bioassay was conducted at 25°C under continuous light (ca. 2000 lux) with lettuce seedlings for three days and rice seedlings (Oryza sativa L., cv. Koshihikari) for seven days. I reduced root elongation of lettuce seedlings by about 40% at a concentration of 100 ppm, but II reduced it by about 80% even at 10 ppm. III reduced the root length of lettuce seedlings to 20% at 50 ppm. However, the inhibitory effect of I, II and III on the hypocotyl growth was not clearly observed at these concentrations. I did not affect growth of rice seedlings even at 100 ppm, while II reduced the total root length by 50% at 50 ppm and by 80% at 100 ppm. A dose of 100 ppm of III showed 80% inhibition of the root elongation of rice seedlings and 40% inhibition on the total length (length of second leaf sheath plus length of second leaf).