Deforestation of riparian zones is known to influence the numbers and kinds of organisms that inhabit adjoining streams, but little quantitative information is available on how much deforestation must occur before the biota isaffected. We sampled fishes and stream habitats in 12 stream segments downstream from deforested but vegetated riparian patches 0-5.3 km long, all downslope from watersheds with at least 95% forest cover. We found an overall decrease in fish abundance with increasing length of nonforested riparian patch; sculpins, benthic minnows, and darters decreased, and sunfishes and water-column minnows increased in numbers. Introduced species were more common downstream from longer riparian patches. Habitat diversity decreased and riffles became filled with fine sediments as upstream patch length increased. Length of upstream nonforested patch and substrate particle size were much stronger predictors of fish occurrence than riparian patch width. Faunal characteristics and physical features of the stream changed in direct proportion to the gradient of riparian disturbance, but the abundance of several species underwent pronounced change at particular threshold patch lengths. These results suggest that riparian forest removal leads to shifts in the structure of stream fish assemblages due to (1) decreases in fish species that do not guard hidden eggs or that are dependent on swift, shallow water that flows over relatively sediment-free substrates, or (2) increases in fishes that guard their young in pebble or pit nests or that live in slower, deeper water. When watershed development is anticipated or planned, limited clearing of riparian trees may cause minor disturbance to the fish assemblage, but streams in even a heavily forested watershed with vegetated riparian buffers cannot tolerate disruption of riparian-zone trees over much more than 1 km in length. Riparian buffer length and area should be given stronger consideration in stream protection and restoration plans.