Ageratum houstonianum (Ageratum, flossflower, blue billygoat weed) is an annual plant that tends to become a pest in gardens and pastures. Clinical signs for A. houstonianum toxicosis in cattle are characterized by either an acute hemorrhagic course or sub-acute photodynamic dermatitis. The toxicosis has often been associated with Holstein-Friesian or crossbreed Holstein cattle less resistant to tropical climate conditions. During a recent especially dry spring about 40 adult Zebu cattle were found dead, while another 40/800 animals were sacrificed. The animals had been relocated to the problem area about 4 mo before, where due to the prolonged drought, A. houstonianum was almost exclusively the only pasture available. The intoxicated cattle did not show the characteristic toxic dermatitis reported for A. houstonianum acute toxicosis; but post-mortem examination revealed bloody serous fluid in coccyx-femoral joints and hemorrhages in the large muscle tissues, while liver, kidney and heart also had hemorrhages. To confirm the toxic plant as cause of the toxicosis, phytochemical Qualitative screening and a novel thin-layer chromatographic characterization of plant extracts were done. The chromatographic profiles of coumarin compounds, alkaloids and triterpens in ruminal and intestinal contents were similar to those obtained from A. houstonianum plants from the same area, confirming ingestion of A. houstonianum as cause of the toxicosis. The coincidence of adverse nutritional conditions together with the cattle's ignorance of the grazing area predisposed the plant toxicosis.