ABSTRACT: The current study aimed to describe the massive mortality of farmed Nile tilapia Oreochromis niloticus associated with the monogenean Gyrodactylus cichlidarum from the northwestern Mexican Pacific coast. The ectoparasite was identified using measurements of the hard parts and compared with previous literature. Approximately 20,000 tilapias were subjected to subsequent losses over a three day period, a total of 2% of the initial seed stocked died due to the presence of this gyrodactylid parasite. The presence of the monogenean was the principal cause for chemical treatments. There is no doubt that infections by parasites have major consequences for species of small fish in culture and must consequently be considered as a fundamental factor within any system of aquaculture. A histo-pathological examination of the gills of fish showed the presence of the monogenean attached to the fi laments was causing hyperplasia. The fish showed no signs of bacteria or virus infection. This study reports for the first time the occurrence of massive mortality caused by this monogenean in a tilapia fish farm in the Mexican Pacific and also extends their known geographical distribution.