On some vertical rocky shores the pulmonate limpet, Siphonaria kurracheensis, had a bimodal distribution, being most abundant at high and low levels of the shore and least abundant at intermediate levels where the density of the acmaeid limpet, Notoacmea onychitis, was greatest. These species overlap considerably in their use of vertical position on the shore. Because individual limpets returned faithfully to home scars at low tide, they probably interacted only with their immediate neighbors. The distance between nearest neighbors increased as the sum of their sizes increased, and interspecific pairs were further apart than pairs of N. onychitis, indicating both intra- and interspecific competitive effects. The effect of N. onychitis on S. kurracheensis was great because following experimental removal of all limpets, the communities of S. kurracheensis established in the absence of the acmaeid limpet, utilized the intermediate as well as extreme levels of the shore. Apparently N. onychitis is a superior competitor for space whose niche is intrusive at intermediate levels of the shore and is contained within the niche of S. kurracheensis.