The hyporheic zones within 16 streams representing five ecoregions of Oklahoma were sampled to describe the variation in composition and abundance of the hyporheos across a large and physically diverse geographic region and to determine those factors responsible for this variation. Sites included gravel-bottomed streams located in the Hot Continental and Subtropical ecoregions in the east and streams with sand substrates in the central and western areas of the state (Prairie, Tropical/Subtropical Steppe, and Temperate Steppe ecoregions). A total of 95 samples were collected by pumping water from narrow wells installed to depths of 30 cm, 60cm, and 100cm below the stream bottom at midstream and at the same depths below the water level within instream bars and on lateral banks. Taxon richness and total abundances were greatest in eastern streams and decreased in streams located in the central and western portions of the state. Cyclopoid and harpacticoid copepods dominated the hyporheos in the eastern streams, whereas Chironomidae and Nematoda were often the only components in some western streams. Major factors influencing the composition and abundance of the hyporheos in Oklahoma streams located in different ecoregions as indicated by ordination and supported by Spearman rank correlations include longitude, sediment size, and dissolved oxygen. Ongoing studies will provide a greater understanding of both spatial and temporal distributional patterns as related to environmental conditions.