Stonefly life histories were studied March, 1987 through May, 1990 in the Rio Conejos, Colorado. Adult presence phenology and intensity were monitored daily in the summers of 1988 and 1989 and were coupled with monthly benthic samples to assess nymphal growth. Eggs of several species were reared. Thirty-one species were collected, with several multi-species assemblages occurring in Capnia, Utacapnia, Taenionema, Suwallia, Triznaka, Isogenoides and Isoperla. Sufficient data were obtained to reveal partial or complete life histories of 13 species, five of which have not been previously reported. New information included the 9- to 10-mo egg diapause and semivoltine life histories of Isogenoides zionensis Hanson, Pteronarcella badia (Hagen) and Pteronarcys californica Newport. Additionally, Isoperla phalerata (Smith) had univoltine-slow growth, and L quinquepunctata (Banks) was univoltine-fast. Previously unstudied emergence periods are presented for Triznaka signata (Banks), Suwallia wardi Kondratieff and Kirchner and S. pallidula (Banks). The later two species temporally segregated emergence in both years. Isoperla fulva Claassen emerged in June, and was temporally segregated from its congeners. Regression analysis of cumulative percentage catch revealed two adult presence patterns. Eight species had slopes <7%/d (extended pattern), and only two, I. zionensis and P. cal'fornica, had slopes >7 %/d (synchronous pattern). Several stoneflies …