Predicting the effects of exotic species on native communities often depends on the characteristics of the recipient community and on the food habits of the invasive. Field experiments assessed the direct and interactive effects of an invasive omnivorous crayfish (Procambarus clarkii) and herbivorous snails (Physella gyrina) or shrimp (Atyoida bisculcata) on stream communities in California and Hawaii. In the Californian stream, snails significantly reduced algae (primarily diatoms) and sediment, but predation by crayfish ameliorated these effects. Only crayfish reduced filamentous algae (Cladophora), and total invertebrates, filter-feeders and chironomids were higher in alone consumer treatments than in those containing both or neither consumer, whereas predatory invertebrates were lower where crayfish were present. In the Hawaiian stream, neither crayfish nor shrimp affected algal abundance; however, both crayfish and shrimp reduced filamentous algae (Spirogyra), and only crayfish reduced leaf litter. Total invertebrates and small herbivores were reduced by crayfish but were unaffected by shrimp, and predatory invertebrates were reduced by both consumers, but their combined effects were less than when alone. These results indicate that the same invasive can generate complex and different effects on disparate systems, probably as mediated through multiple trophic interactions, but that crayfish impacts depend on the characteristics of the invaded community.