Heterophyllaea pustulata Hook. f. (Rubiaceae) is a phototoxic plant. It grows in the Andean area of northwest of Argentina, and it causes significant economic losses in the livestock. This plant induces dermal lesions by photosensitization probably due to its content of photosensitizing anthraquinones. This paper describes an outbreak of poisoning in Corriedale sheepfold, which had an incidence of 49%. Ear skin biopsies and blood samples were collected of six affected animals. Liver enzymes remained within the reference limits. Histopathologically, a deep necrotizing dermatitis was identified in all samples. H. pustulata was identified in the areas of grazing. Anthraquinone concentration in leaves was 0.84% p/p, expressed as rubiadin. All findings allow us to conclude that the diagnosis is a primary photosensitization. Huge regional economic losses could be attributed to H. pustulata poisoning, although its toxicity has been little studied.Facultad de Ciencias Veterinarias