Population dynamics of waterhyacinth, Eichhomia crassipes ((Martinus) Solms-Laubach), and a biological control agent, the weevil Neochetina eichhorniae (Warner), were quantified at a site in Wallisville, Tex., during 1987 and 1988. A decline of waterhyacinth occurred that was most pronounced during the 1988 growing season. Changes included decreases in (1) living above water biomass/m2 (≍−57%), (2) living below water biomass/m2 (≍−39%), (3) number of living leaves/plant (≍−50%), and (4) weight/plant (−40%). Associated with these decreases was a 1.5-fold increase in plant density. The waterhyacinth was severely damaged by N. eichhorniae . Highest numbers of N. eichhorniae adults (>55 weevils/m2) occurred in samples collected 15 September 1987. Decreases in the number of weevils for both sexes followed. Declines approached 8 weevils/m2 per month for the following 5 mo. Numbers of pupae per square meter decreased during 1987 by 7.3 pupae/m2 per month. Larval density increased during the winter months with highest larval density noted during December and January. This peak was followed by decreases in third instars of ≍80%. Increases in the number of adults and third instars occurred during the 1988 growing season, however, pupal density remained low (near 0) thereafter. Environmental factors that contributed to the waterhyacinth declines included a drought, nutrient conditions, intermittent freezing temperatures, as well as sustained N. eichhorniae herbivory.