Tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea) appears to control the riparian shrub saltcedar (Tamarix spp.); however, its use is restricted since the fate and effects of this herbicide in aquatic systems are unknown. Possible tebuthiuron impacts on aquatic production were examined in ten 2846-1 three-phase, open-system mesocosms. Each mesocosm contained sediment, water, algae, micro- and macroinvertebrates and fish, and was open to the atmosphere for gas exchange and colonization by indigenous macroinvertebrates and algae. The following nominal doses of tebuthiuron were used: 0 (control), 10, 70, 200, 500, and 1000 μg 1−1. The 200 μg 1−1 dose approximated the highest concentration of tebuthiuron detected in a water body after experimental application to a watershed. Data generated from all treatment levels were used in tebuthiuron fate analysis and in correlation analysis between the mesocosm variables. The control and the 200 μg 1−1 treatment level were replicated (n = 3) to allow for additional statistical analyses of treatment effects at the 200 μg 1−1 level. The adsorption of tebuthiuron to sediments contained in ten mesocosms was described by the Freundlich equation, x/m = 3.24c0.68. Phytoplankton primary production, chironomid density, and chironomid biomass were negatively correlated with tebuthiuron concentration during peak system productivity. Conversely, no trends were observed at any sample date between an omnivorous fish species and herbicide concentration. At the 200 μg 1−1 dose level, only chironomid density was reduced. Factors responsible for reductions in chironomid density may include 1) a species shift in the 200 μg 1−1 treatment algal assemblages toward a greater percentage of ‘unpalatable’ biomass, and 2) decreased algal productivity and/or an algal species shift in mesocosms receiving dose levels greater than 200 μg 1−1. Chironomid density reduction at the 200 μg 1−1 dose level suggests that deleterious effects may occur in some stream systems exposed to a 200 μg 1−1 tebuthiuron concentration.