"Eukiefferiella" and "Tvetenia" are closely related genera belonging to the subfamily Orthocladiinae within the Chironomidae, a family of non-biting midges. All known larvae in these genera are aquatic, being found predominantly in running water. Most species prefer cold, swift-flowing, well-oxygenated streams. Although larvae and pupae of these genera are commonly encountered in streams surveys, there is little information on their taxonomy and ecology in the North American literature. This bulletin is an attempt to subdivide these large and ecologically diverse genera into 12 species groups (groups of similar species). For each group, photographs and distinguishing characteristics are provided to allow positive group identification of the larvae. Taxonomic and ecological discussions are also provided for each group. An historical review, key to species groups presented, comments on the format used in the bulletin, a cladogram illustrating possible phylogenetic relationships within the genera based on larval morphological characters, and a discussion of the methodology employed (including collecting/mounting techniques) are included. (JN)