During June 1985 triploid grass carp (Ctenopharyngodon idellaVȧl.) were stocked in a large canal (2,000 cfs) to evaluate their potential for controlling hydrilla (Hydrilla verticillataL.F. Royle). Hydrilla biomass decreased from 9.1 kg/m2at the time of stocking to 0.12 kg/m2at the end of six weeks (includes effects of partial drawdown). Larger radio-tagged fish (1,000 g) had negotiated smaller canal culverts and siphons in previous studies, but the smaller fish of this stocking (120 g) have not traveled upstream past any structures with an elevation drop or turbulence. A canal drawdown one week after stocking removed aboveground biomass of plants in shallow water, but the reappearance of small plants in this area may indicate that the fish are less willing to feed in shallow water. Fish should be stocked between structures and at frequent intervals on long reaches of uninterrupted canal. Stocking prior to plant biomass buildup allows faster weed control.