Quantitative aspects of biodeposition by a common salt marsh invertebrate, the ribbed mussel Geukensia demissa, were evaluated in a coastal salt marsh. Seasonal biodepositional rates calculated for the ponded-water and transitional marsh, two habitats in which mussels are typically abundant, were based on 1) mussel density, 2) duration of tidal inundation (time available for suspension feeding and biodeposition), and 3) in situ individual mussel biodepositional rates per season. These rates, compared with both contemporaneous physiochemical deposition and previous measurements of net seasonal sediment accumulation in nearby transitional and ponded-water marsh habitats, suggest that mussel biodeposition can account for nearly all net summer sediment accumulation there and for significant fractions of spring and autumn accumulation as well. Winter mussel biodeposition is negligible. Considerable local recycling of biodeposits and other sediment also is suggested by the study. Mussel biodeposition therefore constitutes a large proportion of the annual marsh sediment budget and is geologically important in the accretion of Georgia salt marshes. Biodeposition probably is an important process in many other salt marshes, as well.