Annual production of lime mud by two genera of red algae and one genus of serpulid worms was estimated for an area of modern lime mud accumulation in Eastern Florida Bay. The red algae Melobesia membranacea and Fosliella farinosa and the serpulid worm Spirobis sp. live as epibionts on the leaves of Thalassia testudinum, the extensive marine grass. The lime mud produced by the epibionts was estimated by quantifying 1) the life span of Thalassia ; 2) the abundance of Thalassia ; and 3) the average amount of epibiont calcium carbonate per blade. The estimate also accounts for both aerial variations in standing crop and seasonal variations in growth rate of Thalassia . The estimated annual production of epibiont carbonate is 118 + or - 44 g/m 2 /yr, over six times more than the estimated production by the green alga Penicillus capitatus from the same area (Stockman et at. 1967). This leads to the conclusion that the epibionts on Thalassia produce significant amounts of lime mud in Florida Bay. This result is close to the published estimate for epibiont production in Jamaica of 180 g/m 2 /yr (Land 1970), but it is significantly less than a published estimate for epibiont production in Barbados (Patriquin 1972). Turtle grass has been around since the late Cretaceous (Eva 1980), and algae most likely has had a longer history. Therefore, epibionts may have been significant contributors since the late Cretaceous.