The influence of environmental variability on body form and tissue structure of Cinachyrella australiensis is reported for populations from three sites within Darwin Harbour, Northern Territory, Australia, that varied considerably in hydrological conditions. External morphology of these sponges differed among sites ranging from typical spherical shapes to flattened forms. A large proportion of dry weight consisted of inorganic matter, i.e. silica spicules, varying between 62.9% and 78.2%. Sites with highest water velocity and sediment size were significantly correlated with sponge populations having the greatest inorganic content and lowest organic cellular content and the thickest oxea. Thicker oxea may in part account for the higher structural content of sponges at these sites. There was no significant difference in oxea length among sites. It is concluded that sponges subjected to highly perturbed environs with large water flow and sedimentation regimes may devote more energy to spicule reinforcement relative to organic content. These robust sponges have the potential to make an important structural contribution to their habitats.