The effects of substrate ingestion on gas-free specific gravity, gaseous lift factor (lung volume), and ultimately buoyancy index are described for larval anurans. Observations on each of Litoria genimaculata and Litoria lesueuri show larvae ingest substrate particles in similar amounts in both the field and laboratory, resulting in a significant increase in gas-free specific gravity. Experiments with Rana septentrionalis and Rana sylvatica show that the increases in gas-free specific gravity varies among different types of substrates. Silt and gravel substrates result in a greater gas-free specific gravity than either sand or detritus. Both species of Rana compensate for the increase in gas-free specific gravity by increasing gaseous lift factor, and the resulting buoyancy indices are similar to larvae tested in bare aquaria. Detailed experiments on R. septentrionalis show that, when larvae are transferred from a bare aquarium to one with silt present, silt is ingested rapidly within 3 h and the digestive tract is filled by 48 h. During this period, the close match between the increase in gas-free specific gravity and the increase in gaseous lift factor, indicates a prompt and on going degree of precision in buoyancy control. Substrate ingestion has a profound effect on gas-free specific gravity and larval anurans have evolved a mechanism to regulate buoyancy.