The surface morphology of the ctenidia of Spondylus spinosus Schreibers, 1793 was studied with light and scanning electron microscopy for comparison with the gill structures of other bivalves. The demibranch of S. spinosus is heterorhabdic, with the principal filaments at the descending lamellae and ordinary filaments at the ascending lamellae. The gill lamellae have a prominent gauze-like structure at their distal part, with numerous groups of eight ordinary filaments. They bear ciliary arrays on their frontal surfaces and ostia at their latero-frontal surfaces. Frequent cirral plates form regular interfilamentary junctions. The description of the gill structure of S. spinosus presented here can be used to derive implications for the correlations among the structure, habitat and mode of life of this species. At a particular stage of its adult life, Spondylus spinosus could be used as a subject for biomonitoring studies in natural and experimental environments.