Fourteen mesocosms were constructed during the summer and fall of 1987 at the Univeristy of North Texas. In June of 1988, a treatment program with the synthetic pyrethroid tralomethrin was initiated. The program featured ten weekly spray applications to the mesocosm surfaces to simulate spray drift and five biweekly applications as a fortified soil slurry to simulate runoff. Including the control, six levels of tralomethrin were established by the treatment regime. Triplicate mesocosms were monitored for the control and a dose representing the estimated environmental concentrations of tralomethrin for loading via drift (68.5 ng/l) and runoff (218.9 ng/l). Duplicate mesocosms were monitored at the other four dose levels. Measured levels of tralomethrin in water samples closely paralleled nominal levels. Tralomethrin residues rapidly declined after applications and no residues were detected in the water 2 weeks after the final application. Physicochemical characteristics of the water were not affected by tralomethrin. Mollusks, fish, phytoplankton, macrophytes, and most larval invertebrates inhabiting the hydrosoil were unaffected by tralomethrin. Tralomethrin did significantly reduce abundances of certain macroinvertebrates and zooplankton, relative to the control. The effect of tralomethrin on Ephemeroptera larvae differed considerably between Caenidae and Baetidae. The abundances of Caenidae larvae in treated mesocosms were frequently detected as significantly less than in the control. In contrast, the abundances of Baetidae larvae in treated and control mesocosms were never detected as significantly different. For zooplankton, the densities of Copepods and nauplii in treated mesocosms were 498each detected significantly less than in the control more frequently than when density comparison were made utilizing Cladocera data. The study design and endpoints measured were more appropriate for assessing structural rather than functional properties of an aquatic ecosystem. The results indicate that tralomethrin can alter the structure of an aquatic ecosystem. The alteration in structure appears to be ephemeral in character and could be more thoroughly investigated in future studies which are narrower in scope and longer in duration.