The pattern of resource allocation in the sponge Crambe crambe (Schmidt) (Demospongiae, Poecilosclerida) in 2 contrasting habitats and in specimens of 3 size classes was studied. The sponge biomass per unit area increased with sponge size in both illuminated and dark environments. An increase with size was also found in the illuminated habitat for the investment in organic matter per unit area This parameter was almost constant among the 3 size classes from the shaded habitat, with values similar to those of the medium-slzed specimens from the lllum~nated environment The amount of sihca per mm2 was higher in the dark site where it proved size-dependent, whereas ~t dld not vary with size in the illuminated habitat Investment in reproduction per unit area was higher in the illuminated wall and was positively correlated with size in both habitats. The amount of calcareous debris included by the sponge during its growth did not vary across size classes in either habitat. All variables related to sponge thickness showed significant differences between the 2 sites studied, while neither the size nor the interaction of habitat and size significantly influenced their values. The specimens were thicker in the illuminated habitat, both in their choanosome and ectosome layers. No variations with size or site were found for the canal system. There was a significant habitat effect on the amount of matrix, which was higher in the illuminated environment. Sponges produced more collagen in the shaded environment than in the illuminated site. This was particularly true for specimens larger than 1000 mm2 (medium and large size classes). As for the number of fibres, there was also significantly more fibre material in the sciaphilous sponges. A significant Interaction was found between habitat and size effects on the number of cells per sponge section. The pattern of resource allocation to the different functions considered was similar in medium-sized specimens from the 2 contrasting habitats except for the number of cells and amount of collagen per sponge section. Small and large sponges, on the other hand, featured the highest between-habitat differences in resource allocation. Large sponges from the illuminated habitat invested relatively more energy in organic matter and less in mineral and collagen structures than their sciaphilous counterparts. Small photophilic sponges invested more in silica than small sciaphilous specimens. Investment in reproduction was notably hlgher In the whole size range of photophilic sponges than in the corresponding size classes from the dark site. Consequently, thls species seems able to shift its resource allocation as a function of size and environmental conditions. We propose an interpretation in terms of a higher competitive pressure in the shaded environment that results in increased investment in defensive/supportive (mineral and organic) structures and a lower investment in somatic growth (organic matter) and reproductive output (larvae).