The modern species concept (Mayr, 1963) stresses the interbreeding within, and the reproductive isolation between, species. However, "interbreeding" is not strictly an intraspecific characteristic since numerous interspecific hybrids have been reported, especially among the temperate freshwater fishes. In this investigation, hybridizing (interbreeding) populations of the peamouth chub, Mylocheilus caurinum (Richardson), and the redside shiner, Richardsonius balteatus (Richardson) from Stave Lake, British Columbia, were studied to determine: (1) if interbreeding between them was resulting in the swamping (lack of reproductive isolation) of their gene pools, and (2) if swamping was absent, what isolating mechanisms were operative? To detect the presence or absence of swamping, two approaches were used. The first measured shifts in means of certain morphological characters for the two species within and outside the area of hybridization. The second determined the frequency of various hybrid generations. If swamping is not occurring, the frequency of hybrid backcross individuals should decrease as backcrossing continues. Both approaches indicated that swamping is absent between Mylocheilus and Richardsonius. In the absence of swamping, isolating mechanisms between the two species were examined. Seasonal, temporal (diel), spatial, and ethological premating isolating mechanisms do not appear to be effective since Mylocheilus, Richardsonius, and their hybrids spawn at the same time and place within Devils Creek, a major spawning area in Stave Lake. Egg and fry survival of crosses involving hybrid individuals were measured under experimental conditions to determine if they serve as post-mating isolating mechanisms. F₁ hybrid males are partially sterile as demonstrated by the poor egg survival of crosses involving them. However, egg survival of hybrid females when backcrossed with Mylocheilus and Richardsonius males was comparable to the parental species' crosses. Thus, fertility is not considered an effective isolating mechanism. The experiments testing the fry survival of the parental species, reciprocal F₁ hybrids, and hybrid backcrosses to Mylocheilus(= B CM) yield a clue to what isolating mechanism prevents swamping. The fry survival of reciprocal F₁ hybrid fry do not differ significantly from Mylocheilus fry. In contrast, the survival of hybrid backcross to Mylocheilus fry is approximately 20 % less than either reciprocal F₁ hybrids or Mylocheilus fry after only 48 days rearing. Additionally, 11. 5% of the surviving BCM fry possess gross abnormalities. Thus, hybrid inferiority is demonstrated experimentally. Circumstantial evidence was also gathered from Stave Lake in 1967 which suggests the inferiority of BCM fry. Relative to Mylocheilus, BCMs were five times more abundant as fingerlings than as adults. Little information was gathered on hybrid backcrosses to Richardsonius. They, appeared scarce as fingerlings as well as adults. / Science, Faculty of / Zoology, Department of / Graduate