The urohyal bone is a single median solid bone with anterior tip generally connected to the ventral hypohyal and to the first basibranchial, and the posterior end attached to the pectoral girdle by means of muscle. This bone has been generally used for age determination, but recently it has been distinguished that its morphology is diverse in different groups of fishes such that it could be used in fish taxonomy. The morphology of urohyal in 12 freshwater fishes from 12 genera, 9 families and 6 orders was been studied. Results showed that the urohyals were varied in ratio of length and width relative to total length and fish head length, ventral shape, presence and number of processes in the anterior part, origin of dorsal keel (distance from anterior tip), groove in ventral portion and some other attributes. In all examined cyprinid fish, the ventral part of the urohyal forms a triangular flat facet and there was no process in anterior part of urohyal. In Salmonidae family the ventral part was rhomboid in shape and its anterior end was not completely forked. There was a spine-like process in dorsal part of urohyal in Cyprinodontidae and Poecilidae families. In Hormuz Cichlid, Iranocichla hormuzensis (Perciformes) and Abu Mullet Liza abu, (Mugiliformes) the ventral part forms a triangular grooved facet and its dorsal part bears a process. In Tenualosa ilisha (Clupeidae), there are two lateral processes in the anterior end. Based on the findings, we prepared a key for identification of some freshwater fishes of Iran for the first time
