We compared the life cycles of Claassenia sabulosa (Banks 1900) and Hesperoperla pacifica (Banks 1900) (Plecoptera: Perlidae) in allopatry and sympatry by studying 4 populations in 3 streams of the South Platte River Watershed, Colorado. We estimated 2-year life cycles for both C. sabulosa and H. pacifica in allopatry and sympatry. Evidence of egg diapausing and extended larval recruitment was noted for both species in allopatry and sympatry. There were no apparent differences in larval growth, larval recruitment, or adult emergence periodicity between allopatric and sympatric populations of C. sabulosa. In contrast, allopatric and sympatric H. pacifica populations exhibited differences in the timing of larval growth, larval recruitment, and adult emergence. We feel that the differences in the life cycles of these populations were likely related to water temperature because a substantial water temperature difference (degree days of 1761 and 2563) occurred between study reaches. In sympatry, H. pacifica adults began emerging 2 weeks earlier than C. sabulosa, when water temperature reached 9°C. The emergence of H. pacifica was synchronous and male biased. The emergence of C. sabulosa was protandrous and male biased and began when water temperature reached 17°C. Claassenia sabulosa emergence was extended and lasted for 6 weeks during July and August. The abundance of C. sabulosa adults during this period corresponded closely to fluctuations in water temperature.