Experiments were conducted under controlled conditions to investigate the apparent allelopathic effects of sudex [Sorghum bicolor (L.) Moench ×Sorghum Sudanese (P.) Stapf, cv. FFR 201] on weed and vegetable species. Allelopathic potential, as measured by radicle elongation of herbaceous indicator species, decreased with increasing sudex age. Greatest potential allelopathic activity of sudex shoot tissue was observed when sudex was collected at 7 days of age. Small-seeded broadleaf species were more inhibited in the presence of sudex shoot tissue than were grass species. Two major phytoinhibitors were isolated from aqueous extracts of sudex shoot material by partitioning with diethyl ether, followed by thin-layer and liquid column chromatography. Phytoinhibitors were identified as p-hydroxybenzoic acid and p-hydroxybenzaldehyde, potentially the enzymatic breakdown products of the cyanogenic glycoside dhurrin. The I50 values of these compounds using a cress (Lepidium sativum L.) seed bioassay were 140 and 113 μg/ml for the acid and aldehyde, respectively. Sudex tissue collected at 7 days of age possessed a greater percentage of these phytoinhibitors on a per gram basis than did older sudex tissue. As sudex tissue age increased, the percentage of p-hydroxybenzaldehyde in ether extracts of tissue also increased, while the percentage of p-hydroxybenzoic acid decreased.