AbstractExamination of samples of deep litter poultry manure which caused growth deformation in vegetable crops resulted in the isolation of a potent phytotoxic compound. The chemical properties and symptoms produced by tomato plants grown under hydroponic conditions, showed that it was not 2,4‐D as suggested by other workers but a nitrogen heterocyclic compound with an attached carboxyl group.Poultry trials have shown that an impurity, 4‐amino‐3,5‐dichloro‐2,6‐lutidine, in the coccidiostat clopidol (3,5‐dichloro‐2, 6‐dimethyl‐4‐pyridinol) causes similar phytotoxicity. Increased potency after poultry ingestion indicates that this impurity is metabolised. The most likely metabolite is 4‐amino‐3,5‐dichloro‐6‐methyl picolinic acid. Chemical and physical data of the metabolite are identical to that of the toxic compound isolated from original manure samples.Detailed symptomology produced by the toxic manure on tomato plants is described.