Completely aquatic marine mammals of the order Cetacea such as whales and dolphins have a reduced or absent olfactory system and neither a vomeronasal organ nor an accessory olfactory bulb. In comparison, seals, which are only partially aquatic, have olfactory and accessory olfactory systems including the vomeronasal organ. Thus, there seems to be a strong relation between the degree of adaptation to an aquatic environment and the degree of reduction in olfactory structures. Sirenians, such as manatees and dugongs, are another family of marine mammals which have secondarily adapted to a fully aquatic existence, yet there is dispute about the status of their olfactory structures. In the present study there was no evidence for a vomeronasal organ in the adult West Indian Manatee, Trichechus manatus. Additionally, the main olfactory system appeared quite rudimentary. These observations support the hypothesis that, in mammals, secondary adaptation to an aquatic environment leads to the reduction or loss of the olfactory senses.