The physical environment and patterns of distribution and abundance of macroalgae, sessile and mobile animals are described for the littoral zone of Taboguilla Island, Bay of Panama. The substratum is basaltic and heterogeneous in both texture and topography. Some physical factors vary seasonally. During the wet season, (May to mid-December) rain is frequently heavy, daily air temperatures range from 24 to 31.5°C, surface sea-water temperatures range from 26 to 28°C, and wind is usually low but variable in speed and direction. During the dry season (mid-December through April), rain is infrequent, daily air temperatures range from 22 to 32°C, surface sea-water temperatures range from 18 to 27°C, wind ranges from northwesterly to northeasterly and can reach 30 knots. Water clarity is usually poor throughout the year. Taboguilla shores appear barren throughout the year because benthic animals and erect macroalgae are rare on exposed rock surfaces. In the high zone most surfaces are bare (91.5 to 98.1%), with small barnacles Chthamalus fissus (Darwin), Euraphia imperatrix (Pilsbry) the dominant space-occupants. In mid and low zones, encrusting algae dominate space (25.9 to 92.5% cover) and sessile animals are scarce (< 1 to 9.8% cover). Maximum cover (7.0%) of erect algae occurs in the low zone. The plants are short, < 5 cm tall. Consumers (e.g., limpets, predaceous gastropods, crabs, chitons, fishes) are diverse and abundant in all zones. Both prey and consumer species composition and abundances change from high to low zones. Densities tend to vary more in the high, and less in lower zones. The large herbivorous crab Grapsus grapsus (L.) is usually most abundant at higher intertidal levels. In contrast, fishes are probably effectively denser in lower than in higher zones, at least at high tide, although many range throughout the intertidal as well as the subtidal zones. Both species richness ( S) and diversity ( H′) increase with decreasing tidal level. Despite the seasonal changes in the physical environment, seasonal changes in community structure are small or lacking. Annual changes are sometimes larger, but still small in comparison to temperate regions. That crustose algae dominate this otherwise relatively barren shoreline seems due to: (1) intense, consistent grazing and predation by a diverse assemblage of vertebrate and invertebrate consumers on the upright algae and sessile animals, (2) desiccation, possibly ultraviolet and heat stress, especially at higher tidal levels, and (3) possible inhibition of settlement by crustose algae.