Description: C. acuminatus and C. superciliosus (L.) are the dominant resident fish of high shore pools in the rocky intertidal of the western Cape Peninsula, South Africa. The factors influencing pool choice were experimentally assessed to elucidate the mechanisms by which the two species partition the resources of the intertidal environment. A total of 215 5 fish were tested in extensive laboratory studies, running from 02. J une 1997 to 19. December 1997. An artificial tidepool system was erected to examine preferences of fish for tidepools equipped with different microhabitats and to examine effects of intra- and interspecific interactions on pool choice. C. acuminatus displayed a strong preference for artificial pool equipped with rock cover, and in particular, crevices with an interstitial space of 1 cm, a space which matches the body size. C. superciliosus, however, was less restricted to a specific cover type. Both species made the selection of a particular substratum almost entirely visually, rather than through the use of chemical or tactile cues. Furthermore, the position of shelter in respect to the pool bottom was found to be important, with shelter resting on the bottom being preferred to shelter raised off the pool bottom. lntraspecific territoriality by C. acuminatus was absent, whereas in C. superci/iosus agonistic behaviour appeared to result in juveniles being forced to occupy less favoured areas within the pool. Interspecific agonistic behaviour in terms of occupation of specific pools by certain individuals was found to be unimportant in regulating fish densities, since both species can be found together when shelter is limited. Recapture of tagged fish released in the pool of capture confirmed Iaboratory observations and indicated that C. acuminatus and C. superciliosus do occupy discrete low tide habitats when immersed. Those experiments were conducted from 02. October 1997 to 16. December 1997 at the natural intertidal zone of the western Cape Peninsula. The length of time individual fish stayed in pools varied considerably within species. C. superciliosus had a short average duration of stay (four days). In contrast, C. acuminatus showed a high fidelity to the pool where it was first marked (14 days). The comparison between pools which were sampled at the same time intervals indicated that site fidelity and the expansion of the pool population depend upon local topographic characteristics and morphological stability of pools. The general pattern of recaptures observed suggested that both species move over a home range, with the number of suitable pools within this range being less for C. acuminatus than for C. superciliosus. The observed differences in the pattern of pool selection of the two species may be explained in terms of their swimming ability and feeding behaviour. The ecological differences between the two species indicate that C. superciliosus is the most generalised of the South African clinids, while the specialisations exhibited by its congener, C. acuminatus, allow the two species to coexist with limited competition.The observed differences in the pattern of pool selection of the two species may be explained in terms of their swimming ability and feeding behaviour. The ecological differences between the two species indicate that C. superciliosus is the most generalised of the South African clinids, while the specialisations exhibited by its congener, C. acuminatus, allow the two species to coexist with limited competition.