Water hyacinth (Eichhornia crassipes (Mart.) Solms.) creates severe problems in the irrigation districts of Mexico, particularly in western Sinaloa. Therefore water hyacinth weevils (Neochetina eichhorniae Warner and N. bruchi Hustache), imported from the USA in 1993, were used to initiate a biological control program. Precautionary screening revealed that some were infected with a microsporidian so disease-free colonies were produced by eliminating infected breeding lines. To demonstrate effectiveness prior to open field releases, weevils were first released in cages at field sites. Weevil intensity increased to 6.3 weevils/plant after 320 days when the plants were all dead or dying. More than 8,600 N. bruchi and 14,500 N. eichhorniae were then released at various sites during January 1995 to August 1996. Waterhyacinth coverage declined at Batamote reservoir (134 ha) from 95% to <3% by 1997; at the 12-ha Hilda reservoir from 100% in May 1995 to 1% by March 1998; at the 42.3-ha Arroyo Prieto reservoir from 100% to 1% during the same interval; and at the Mariquita reservoir (492 ha), the largest reservoir in the Humaya system, from 394 ha (80%)to 98.4 ha (20%).