The ecological function of the secondary metabolite amphitoxin produced by Callyspongia (Euplacella) biru is evaluated by a forced confrontation with the free-living scleractinian coral Fungia fungites at different exposure times. Our major goal was to determine whether such a forced confrontation with a spatial competitor would cause a significant change in the concentration of amphitoxin in the sponge tissue, indicating a regulatory mechanism of amphitoxin production. Firstly, the concentrations of amphitoxin of sponge fragments submitted to forced confrontation with a mushroom coral did not differ between the exposure times. Secondly, all sponge fragments, including the controls had a consistently lower amphitoxin concentration during the course of the experiment than the natural variation. Thirdly, the concentration of amphitoxin varied substantially between specimens implying that other ecological factors besides spatial competition regulate the production of bioactive compounds of C. biru.