The hypothesis that Arizona and brown trout regulate their numbers through the emigration of all individuals that are in excess of resources was tested experimentally. Different amounts of food and cover were provided to mixed sizes of trout in artificial stream channels with escape routes. The number of both species of trout that became resident generally varied in direct proportion to the amount of cover present; the numbers that established residency in either species stabilized in response to cover levels within 6 days. Emigratory behavior was much slower in response to food than to cover. The smallest Arizona trout showed the greatest decrease in numbers that established residency after being starved for 10 days; however they did not increase their numbers when food was increased above 30 g/day. Arizona trout over 14 cm SL and brown trout over 11 cm SL showed no change in the number of residents whether they were fed or starved for 69 days. There were consistent numbers of both species within 2 cm size intervals that became resident, suggesting that each size class independently regulates its numbers to its own resources. The small inter-replicate variance in numbers of trout that became resident at different levels of resources supports the hypothesis that Arizona and brown trout use emigration as a behavioral mechanism for self-regulation of numbers. The numbers of trout that became resident in channels with coexisting species were close to those observed in single species tests wherein resources were the same; immigrant brown trout were able to induce emigration in resident Arizona trout whereas immigrant Arizona trout seldom displaced resident brown trout. Therefore, theories concerning self-regulation of population numbers should be expanded to include an interspecific mechanism. Management practices such as stocking programs, habitat manipulation and multi-species fisheries can be ineffective unless the ramifications of population regulation are included in their design.