Experimental manipulations in the rocky intertidal have demonstrated that in the absence of predation or physical disturbance (Dayton 1971) a competitive dominant species can monopolize the primary space, thereby excluding competitors. In the paper the effect of one such interaction, the colonization of secondary space, on the population size and dynamics of an inferior competitor is examined. The mussel Mytilus californianus is the dominant competitor and barnacles the inferior competitors for primary substrate in exposed rocky intertidal systems in the Northeast Pacific. However, dense populations of barnacles growing on mussels in intertidal communities were frequently observed. This observation suggested that barnacle populations might not be reduced, and might actually be enhanced, by their dominant competitor.