Dispersion patterns exhibited by a population of larvae of black flies consisting ofCnephia dacotensis(Dyar and Shannon) andSimulium rostratum(Lundström) are described. Groups of larvae of 5.venustumSay/verecundumStone and Jamnback, the species complex to whichS. rostratumbelongs, are known to exhibit a spaced dispersion pattern when found in groups consisting of only that species complex. Groups of only larvae ofC. dacotensis areknown to exhibit a banded or clumped dispersion pattern. When larvae ofC. dacotensisandS. rostratumwere found together they exhibited a spaced or a clumped dispersion pattern. Groups exhibiting a spaced dispersion pattern were found in microhabitats with lower water velocities (46.5 ± 4.6 cm s‐1), and with a lower percentage ofC. dacotensis(14.9%). Groups exhibiting a clumped dispersion pattern were found in microhabitats with higher water velocities (78.5 ± 5.2 cm s‐1), and with a higher percentage ofC. dacotensis(80.5%). It could not be determined if water velocity or percentC. dacotensisdetermined dispersion pattern. Bands formed by groups of onlyC. dacotensisare also described. These bands were perpendicular to the mean direction of flow, two to three larvae thick, and up to 30 cm long. They were 1.9 ± 0.16 cm ([xbar] ± S.E.) (n = 18) apart. The arrangement of bands on the substratum changed from day to day.