Forty-five species of sessile animals, including 32 species of cyclostome and cheilostome bryozoans, settled on experimental panels over a 1 yr period. Colonies were readily overgrown by tubes of the serpulid Pomatoceros triqueter (L.) which became the dominant species on the panel community. However, a number of bryozoan species avoided competitive exclusion by settling preferentially or by growlng epizoically onto the tubes which were, therefore, acting as spatial refuges from overgrowth by the serpulid. Since bryozoans are inferior interference spatial competitors but are known to have greater longevity than P. triqueter, vegetative growth of the surviving colonies and subsequent pre-emptive exploitative competition are probably the main processes which give rise to the replacement of the serpulid to produce the bryozoan-dominated communities on nearby boulders.