Lagoon molluscan assemblages at Fanning Island are described in terms of three topographical areas: the lagoon reef flat, the patch reefs, and the lagoon floor. Among the large mollusks, Clypeomorus brevis, Rhinoclavis asper, Pupa sulcata, Pyramidella sp., and two bivalves, Fragum fragum and Tellina robusta, are the principal components of the fauna of the reef flat; Cypraea moneta and Trochus histrio are the dominant epifaunal mollusks of rubble on patch reefs; and sessile bivalves, Cardita variegata, Electroma sp., Ostrea sandvichensis, and Tridacna maxima, are associated with coral. The micromolluscan assemblages of the lagoon reef flat are dominated by Tricolia variabilis, and patch reef and lagoon floor assemblages by Diala flammea. Obtortio sulcifera is the second most abundant mollusk on the patch reefs and O. pupoides the second most abundant mollusk on the lagoon floor. The patch reef and lagoon floor assemblages are distinguishable into assemblages associated with turbid water and clear water areas of the lagoon. Standing crops of micromollusks are greatest on the windward or southeastern periphery of the lagoon reef flat. The lagoon mollusks are distinguished from the seaward reef mollusks in terms of species composition, modes of life, and feeding habits. The lagoon assemblages are predominantly herbivores and suspension feeders among the macrofauna, and are epifaunal herbivores among the microfauna. The seaward reef macrofauna is dominated by carnivores and herbivores, and the microfauna by faunal grazers. Standing crops of seaward reef micromollusks are less than those in the lagoon and the species diversity index is higher.