Three adult crossbred Thoroughbred horses died in June 2018 at a ranch near Hamedan, Iran. Rigor mortis was evident with no signs of post-mortem bloating in the carcasses. The neck muscles appeared excessively cyanotic in necropsy. In addition, widespread petechiae were observed on the inner surface of the thoracic intercostal muscles. Dropsy was visible in the inner surface of the abdominal wall and peritoneum. The stomach content was watery and dark gray. Furthermore, gastric mucosa was also gray. The livers had a dark appearance and the adjacent duodenum was gaseous and blackened. The kidneys showed autolysis with areas of medullary hyperemia. In addition to decomposition, a specific odor similar to the garlic aroma or natural gas could be smelled from the carcasses. Tissues and contents of various organs, as well as food and water samples, were collected for diagnostic tests. Preliminary toxicological studies showed detectable amounts of phosphide in the samples collected from gastric contents of the animals, and complementary tests confirmed the presence of zinc phosphide in the concentrate feed.