This is the first report of the sipunculan Aspidosiphon (Aspidosiphon) elegans (Chamisso & Eysenhardt, 1821) in the tropical eastern Pacific. With this species the number of coral borers rises to 18 for this region. The upogebiidid crustacean Pomatogebia rugosa (Lockington, 1878) was reported previously (as Upogebia rugosa) from coral colonies in the Gulf of California, Mexico, and from coral reefs of Golfo Dulce, Costa Rica; the latter represented a southward range extension of approximately 3500 km. Subsequently, P. rugosa was recorded from branches of Pocillopora corals in Colombia, extending the range farther southward. In our study, both species were extracted from colonies of the massive coral Porites lobata Dana from Golfo Dulce, southern Pacific coast of Costa Rica. Aspidosiphon (A.) elegans ranged in length from 1 to 20 rom and was present in a density as high as 300 individuals per 1000 cm3. Pomatogebia rugosa was present in 14% of the colonies examined and was responsible for 0.6 ± 0.35% of the CaC03 removed at one site in Golfo Dulce; at her-site-it-was_presenLin_3J.£. _ 2.5 ± 2.22% of the CaC03 removed. P. rugosa was found living in pairs insi live coral colonies of Porites lobata, in branched tunnels about 2.5 rom in diameter and lined with mud. Bioerosion caused by these two species of borers in the eastern Pacific is minimal compared with that caused by sea urchins and boring bivalves.