The Heptageniidae is one of the largest mayfly groups containing about 400 species. Since the larvae are dorso - ventrally flattened in body shape, they are mainly adapted to clinging or spawling on rocks in riffles of streams. The larvae of Heptageniidae can be distinguished from other families by their distinctly depressed or flattened body and by their prognathous head with eyes and antennae dorsalIy. The head capsule is complete dorsally. Abdominal gills are present on segments 1 - 7 consisting of single semi - oval or rarely slender lamellae, usually with fibrilliform tufts at or near the base. The study on Heptageniidae was conducted from main streams in Hoang Lien National Park, Lao Cai province during 2007-2010. Field investigations for quantitative (Surber net 50 cm x 50 cm, mesh size 0.2 mm, riffle and pool) and quantitative sampling were conducted at 9 sites (altitude from 79 m to 2040 m) along maih stream. The results shows a total of 18 species belonging to 8 genera of Hetageniidae. Seven species are new for the area: Afronurus mnong Nguyen and Bae, 2003
Ecdyonurus landai Braasch and Soldan, 1984
Epeorus hieroglyphicus Braasch and Soldan, 1984
Paegniodes dao Nguyen and Bae, 2004
Thalerosphyrus separatus Nguyen and Bae, 2004
Ecdyonurus sp. and Epeorus sp. The heptagenial species are diverse at high altitudinal areas than in lower altitudinal areas of the stream system. Riffle habitats generally yield approximately twice the number of species and individuals that are collected in pool habitats.