Effects of littoral periphyton community structure upon substrate colonization by chydorids and chironomids were examined in a hardwater, eutrophic lake (Douglas Lake, MI) and a softwater, oligotrophic lake (Lake Lacawac, PA). Periphyton species composition and standing crop were manipulated by enrichment with N, P, and bicarbonate (C) using nutrient-diffusing clay flower pot substrates. Pots diffusing 9 combinations of N and P were deployed in Douglas Lake, and supported distinctive periphyton communities which ranged in standing crop from 80000000 um3/cm2 to 1330000000 um3/cm2 total biovolume, and in dominance from diatoms to filamentous green algae. In Lake Lacawac, pots supplying 5 combinations fo N, P and C were colonized by periphyton which ranged from 700000000 um3/cm2 to 7800000000 um3/cm2 in total viobolume and dominance from filamentous greens to green colonies and unicells. Eleven species of chydorids were collected in Douglas Lake, and ranged from 1.5-47.1% of total invertebrates according to nutrient treatment. Of these, Alona setulosa, A. affinis and Alonella nana responded positively both to the growth of green filaments and to increases in diatom abundance. The chironomid community comprised 33.8-73% of total invertebrates. Of 9 taxa counted, the freeliving species Corynoneura nr. lobata showed the greatest response to changes in the periphyton community, increasing in both density and mean body size on substrates dominated by green filaments. In Lake Lacawac the chydorid community of 17 species and chironomid assemblage of 13 enumerated taxa showed only slight response to changes in periphyton standing crop and shifts in species dominance from Oedogonium sp. to Cryptomonas sp. and an unidentified coccoid chlorophyte. Invertebrate motility and the growth forms of key algal species appeared to be important features of chydorid and chironomid colonization of littoral periphyton in the two lakes.