We studied the effects of grazing by the mayflyStenonemasp. and the caddisflyPsilotretasp. on periphyton communities at two sites in the field, and we manipulated grazer densities in laboratory experiments to measure grazing effects on the periphyton community. In the field, periphyton abundance on unglazed clay tiles was compared between tiles elevated above the stream bottom to excludePsilotretasp. from the grazing community and tiles placed directly on the stream bottom allowing access by all grazers. There was no significant difference in ash free dry mass (AFDM) or chlorophyllabetween these two treatments at either site, suggesting thatPsilotretasp. had little affect on the periphyton community. In the laboratory, two sets of experimental stream chambers containing tiles with natural periphyton assemblages were used, and grazer densities within the chambers were varied to reflect the different densities of grazers at the field. After two weeks, neitherPsilotretasp. norStenonemasp. showed significant weight changes. However, survivorship ofStenonemasp. was inversely related to grazer density, suggesting that inter- or intra-specific competition may have existed at the highest grazer densities. Both AFDM and chlorophyllawere significantly higher in control chambers than chambers with grazers; also, there were no significant differences in AFDM or chlorophyllaconcentration among the different grazer density treatments. Thus, unlike the field exclusion studies which showed no impact ofPsilotretasp. grazing on periphyton, laboratory studies showed pronounced grazing effects. These results suggest that our laboratory stream chambers may have influenced biotic (predation/competition) and/or abiotic (flow regime, etc.) factors allowing grazers to show more dramatic impacts on periphyton than occur in the natural environment. These results suggest that caution should be taken when extrapolating grazing effects observed in the laboratory to a field situation.