The holomorphology of ll life stages of 20 western Nearctic Isoperla and one Cascadoperla was studied over the 3-year period 1975-1978. One monotypicgenus new to science, Cascadoperla, is described, and Cascadoperla tricture (Hoppe) designated as the type species. The nymph, adult male and female, and ova are described and illustrated.

Distributional data and taxonomic keys art presented for thirty-four species of Plecoptera known to occur in Oklahoma. Ten species are new records for the state. Descriptions are provided for two species new to science, Zealeuctra cherokee and Isoperla brevis, and for the previously unknown male of Strophopteryx cucullata Frison and female of Helopicus nalatus (Frison).