Interference competition for spawning space, and competition for space between underyearlings, was studied in sympatric populations of brown and rainbow trout using Scotts Creek, the major spawning tributary of Lake Alexandrina, between November 1979 and September 1983. Redd superimposition severely limited the spawning success of both species and favoured late spawning rainbow trout. The overall spawning success, from egg deposition to fry emergence, was 2.1% for rainbow trout and 0.2% for the earlier emerging brown trout. Redd superimposition caused a 94% reduction in the spawning success of brown trout in an experimental spawning section of Scotts Creek. Spawning of female rainbow trout was investigated with simple models to aid in understanding the relationships between arrival pattern of females, capacity for spawning space and associated Redd superimposition, and pattern of fry emergence. The potential for competition for spawning space to influence timing of runs through selection acting on time of spawning was also considered. The potential for competition for space between underyearlings in Scotts Creek was determined from an investigation of social interactions and microhabitat partitioning in stream observation troughs. Species and size appeared more important than prior residence in governing dominance relationships. Rainbow trout were socially dominant as fry in riffles, and after the fry stage brown trout were socially dominant in all microhabitats tested. Social conflict between fry and fingerlings was minimized by size dependent aggression, aggression being highest between fish of similar size. Study of comparative abundance, migration and population dynamics of 0+ brown and rainbow trout in Scotts Creek provided information for assessing the importance of competition between underyearlings in the stream and its role in regulating populations of the two species in Lake Alexandrina. The juvenile salmonid populations in Scotts Creek were dominated by late emerging 0+ rainbow trout despite a much greater tendency shown by brown trout to remain in the stream following emergence. Rainbow trout juvenile output from Scotts Creek was heavily dependent on recently emerged fry whereas that of brown trout was dependent more on fish which had undergone a period of stream residence. Competition between juvenile brown and rainbow trout was discussed in relation to the migratory strategies employed by each species with respect to lotic versus lentic rearing. Competition between brown and rainbow trout was discussed in the context of competition and niche theories. Hutchinson's multidimensional hypervolume concept of the niche was shown to be inadequate for species such as freshwater fish which have multistage lifecycles. A modification to Hutchinson's model is proposed which takes into account the entire lifetime of a species with population regulatory mechanisms, including competition, potentially acting at various stages of the lifecycle. The possibility of competitive exclusion of brown trout from Lake Alexandrina by rainbow trout is considered, and factors enabling coexistence in the face of severe competition for spawning space are discussed.