Competition is defined as the demand of two or more organisms for the same extrinsic resources in excess of supply. The distribution, movements, behaviour and food of trout and shiners in Paul and Pinantan lakes were studied in order to determine the item's and mechanisms of interspecific competition between them. Data from recent years were compared with data for years when trout alone inhabited the lake. No interspecific aggression was observed. The possibility that the two species were competing for space was discounted. Stomach contents of shiners in Pinantan Lake revealed a marked qualitative diurnal food cycle. In Paul Lake, shiners have drastically reduced the Gammarus population relative to its pre-shiner abundance. This overgrazing was caused by the concentration of large numbers of shiners over the shoals where Gammarus are also present in their highest concentrations and the ability of shiners to pursue food deeper into the weeds and to graze an area more thoroughly than trout. In Pinantan Lake shiners have apparently-reduced the density of Daphnia to a point where trout are unable to feed on them as rapidly as in pre-shiner years. The ability of both species to utilize many types of food tends to reduce the intensity of competition. The study demonstrates how false implications may arise from a delayed appraisal of competition. If observations had not been made on Paul Lake until after competition had been observed the importance of Gammarus as an item of competition would have probably been overlooked and the whole competitive relationship misconstrued. Included among the basic mechanisms of competition is the consumption by one or more organisms of something in short supply before it reaches a potential habitat where it would become available to another organism or group. Environmental factors and behaviour were shown to be important influences in the dynamics of competition. The physical and biological environment and the distribution and behaviour of competitors may be in states of continual flux. Hence natural competitive relationships can be considerably more complicated and variable than situations described by the most elastic of theoretical models. / Science, Faculty of / Zoology, Department of / Graduate