The Mink and Valley rivers, Manitoba, contain two sympatric species of dace (Cyprinidae: Rhinichthys). These fishes, although interfertile, have never been reported to hybridize with each other in nature. Seasonal isolation is only partially developed, since overlap in time of spawning exists. During spawning, adults of the two species tend to occupy different habitats, the longnose dace, Rhinichthys cataractae, being most abundant in water velocities faster that 45 cm/sec and the blacknose dace, R. atratulus, in water velocities less than 45 cm/sec. But some overlap exists. However, the place of spawning of the two species is distinctly different. Nests of longnose dace were found in water velocities exceeding 45 cm/sec with large stones (upper limit greater than 5 cm) while nests of blacknose dace were found in slower water velocities with small stones. The patterns of spawning behaviour of these two cyprinids reveal marked differences and obvious incompatibilities. Males of both species actively defend territories prior to spawning but observations reveal distinct preferences for homospecific females over heterospecific females. Observations further indicate that females are receptive only to patterns of courtship behaviour shown by males of their own species. The combination of different places of spawning and patterns of behaviour is considered to be a very powerful barrier to interspecific hybridization. Sperm longevity of both species is of considerable duration (50% motility after 31-32 sec) and could, in the absence of other reinforcing mechanisms, permit accidental fertilization. However, the interplay of the pre-mating isolating mechanisms would appear to effectively maintain a high degree of reproductive isolation.