Nymphal gill presence, location (gill formula) and structure of representative species of the 95 extant North American Plecoptera genera were comparatively studied. Gills or osmobranchiae occurred variously at 10 defined topographic locations on the head, thorax and abdomen in the 41 gilled genera. Each separate gill is described, illustrated, and abundance and location of chloride cells are discussed. Three families, Capniidae, Leuctridae and Chloroperlidae lack gills, and the Pteronarcyidae and Perlidae have the most complex, branched gills. Number of gill filaments of the posterior sternal₃ gill of mature Pteronarcella badia nymphs varied from 15-32; intra- and interpopulation variation was approximately equal. Number of gill filaments of the anterior supracoxal₁ and posterior supracoxal₃ gills of mature Perlinella drymo nymphs from 9 localities in 4 states varied from 11-26 and 11-28 ( ± SD = 16.8 ± 2.5 and 20.2 ± 3.2, respectively). There was a significant increase in gill branching and number of gill filaments as nymphal size increased in Neoperla clymene from the Brazos River, Texas. A new theory of gill evolution in Plecoptera is proposed, which assumes a gill-less grylloblattid-like orthop-teroid ancestor.