The sediment surface within the Osbornia belt of a mangal on Pulau Hoga, Tukang Besi Islands, Sulawesi, Indonesia, is dominated inter alia by deposit-feeding gastropod molluscs at a mean density of some 230 ind m−2 although, unusually, species of Cerithidea do not occur. Densities of the two most numerous species, the potamidid mud-whelk Terebralia sulcata and the Cerithidea-like cerithiid mud-creeper Cerithium coralium, are inversely correlated, although the species occurred together in 42% of quadrat samples. Within and beyond the normal range of field densities of each species (C. coralium mean 153 m−2; T. sulcata mean 75 m−2) there was no evidence of intraspecific depression of feeding rate, as assessed by the production of faecal pellets, although this was significantly reduced in the occasional very high density aggregations of C. coralium (>1100 m−2). The presence of the larger T. sulcata did appear to have a strong inhibitory effect on feeding in C. coralium; the converse, however, could not be demonstrated. Abundances of Terebralia palustris and C. coralium were also inversely correlated where the two co-occurred.