The intertidal m o l l u s c a n fauna on a sandy-mud beach in Newport Bay, California, USA, is divisible into two v e r t i c a l l y distinct species assemblages which correspond to the m i d l i t t o r a l zone and sublittoral fringe observed on rocky coastlines and exposed sandy beaches. The species assemblage comprising the m i d l i t t o r a l zone is unusual, however, in that the numerically dominant species are not confined to this zone but range downward through the sublittoral fringe. The two species assemblages are v e r t i c a l l y separated at +0.5 to +1.O ft (+0.15 to +0.30 m; relative to O.O tidal datum at mean lower low water), which is higher than p r e v i o u s l y observed for the separation between the m i d l i t t o r a l zone and sublittoral fringe on rocky shores and exposed sandy beaches. The species composition of some of the samples at +0.5 and +1.O ft (+O.15 and +0.30 m) was intermediate between samples higher and lower on the beach, while several samples were devoid of molluscs altogether. Additionally, the surface sedimentary environment changes in terms of an increased percent silt-clay fraction and higher sorting coefficients below +0.5 ft (+0.15 m). Since infaunal zonation is correlated with tidal height at the substrate surface, environmental factors operative at the surface are probably most important in influencing the zonation on this beach.