Inputs of inorganic nutrients in 1992 from seabirds (maximum of ca 80 000 breeding pairs) are compared to inputs from humans (97 700 person days) on Heron Island at the southern end of the Great Barrier Reef, Australia. Noddies deposited ca 107 t, and shearwaters ca 22 t, of fresh guano. The composition of fresh noddy guano was 7.3% N, 1.5% P, 60% moisture and 31.2% other substances. Assuming a similar composition for shearwaters, the total annual deposition of guano contained 9.4 t nitrogen and 1.9 t phosphorus. Although inputs of N and P from human sewage into the cay system (ca 0.3 t of each) were much less than that from birds, all nutrients from humans are released in liquid form and percolate directly into the cay. Nutrients may be transported from guano on Heron Island into the water immediately surrounding the island. -from Authors