Skeletal carbonate sediments on the lagoon floor of Davies Reef (Great Barrier Reef, Australia) are subject to intense attack by microboring organisms, principally algae. The rate at which this microboring process dissolves carbonate was measured experimentally. Fresh, unbored, molluscan sand samples placed in the shallow lagoon (5 m water depth) experienced a weight loss of about 0.9 g/30 g sample (or 3%) in one year due to dissolution of carbonate by microborers (revealed by direct measurement of weight change and point-count analysis). This figure may be converted to about 350 g CaCO 3 dissolved/m 2 lagoon floor/year, which is equivalent to between 18 and 30% of the sediment influx rate to this lagoon, averaged over the past 9,000 years. Consideration of the experimental design suggests that the experiment underestimated the true rate of microboring in lagoon floor sediments. We conclude that the extent of microboring in carbonate sands may provide information on the rate of sediment deposition, and that dissolution of carbonate sediments by microborers is a significant factor in whole-reef CaCO 3 budgets.