The association between a syllid polychaete and the marine sponge Ianthella basta (Pallas, 1776) is described from the island of Guam (Micronesia). The polychaete is a new species of Haplosyllis Langerhans, 1879, named H. basticola. The new species was observed infesting in extraordinary high numbers the inside of the sponge canals and mesenchyme without any specially built tube-like structure. The closest species to H. basticola is H. anthogorgicola Utinomi, 1956 but they clearly differ in size, morphology of the simple setae and the annulations of the dorsal cirri. Haplosyllis basticola, n. sp. is characterized by a maximum of 25 setigers, dorsal cirri of the anterior 1st to 4th setigers clearly annulated averaging 10, 3, 3, and 4 annulations respectively, and the presence of only one simple seta by parapodia with a large subterminal tooth and two smaller terminal teeth. A size frequency histogram of the population showed a unimodal distribution, which could be an indication of a continuous reproductive effort. The presence of female and male reproductive stolons gave us the opportunity to describe them. The chemical analysis of worm and sponge extracts and a Nuclear Magnetic Resonance Spectroscopy analysis done on H. basticola failed to confirm the presence of bastadins in the worm. We discuss evidence of a positive reciprocal relationship between the worm and the sponge.