The relationships between the tropical spongeAnomoianthella lamellaPulitzer‐Finali & Pronzato, 1999 (from Papua‐New Guinea) and its endobionts, the barnacleAcastasp. and the polychaeteHaplosyllis spongicola(Grube, 1855), were analysed using optical and scanning electron microscope techniques. The specimens ofAcastasp. live in the mesohyl, close to the inhalant surface of the sponge, where they reach a mean density of 12.7‐10 cm‐2. Their apical region is delimited by a thin layer of sponge collagen, whereas sponge cells (archeocytes and lophocytes) gather along the rest of the body of the endobiont. While the barnacles adhere to the spongin skeleton network, the polychaetesH. spongicolaare esclusive to the exhalant canals of the sponge aquiferous system, where they reach a mean density of 57 individuals‐cm"2. No evidence of sponge reaction toH. spongicolawas observed. Potential benefits for the barnacle are protection, against predation and mechanical disturbance, and trophic advantages. Conversely, the barnacle may contribute to strengthen the sponge skeletal network. The association with polychaetes can be regarded as a simple commensalism‐parasitism relationship.