Zebra mussels (Dreissena polymorpha) are beginning to colonize smaller inland lakes and streams in the eastern United States. To test the hypothesis that crayfish (Orconectes rusticus) could significantly reduce the recruitment of zebra mussels in streams, we conducted a replicated (n = 7) crayfish enclosure-exclosure experiment over 15 wk to simulate conditions during the early stages of a zebra mussel invasion. Four male O. rusticus were placed in each of seven enclosures (0.2 m²) each paired with an exclosure (week 0) in a small midwestern lake-outlet stream (mean width, 20 m) before zebra mussel settlement. At week 15 zebra mussel densities were 58% lower in enclosures than in exclosures (P < 0.05). Size frequency distributions did not differ significantly between treatments (P = 0.842; size range 1–10 mm in enclosures, 1–11 mm in exclosures). Densities (no/m²) of other macroinvertebrates did not differ significantly between enclosures and exclosures. Periphyton biomass was not significantly different between treatments, but macroalgal (Chara Nitella) biomass was present in exclosures [9 ± 7 g/m² (mean ± 95% CL)] and absent from enclosures (P < 0.001). These results suggest that streams with moderate to high densities of crayfish will experience slower invasion by zebra mussels, lower final densities of zebra mussels and lower abundance of macroalgae.