Observation on the interspecific competition between Callyspongia aerizusa sponge and scleractinian coral was done at Pramuka Island, Kepulauan Seribu National Park Jakarta by SCUBA diving. It was observed that Callyspogia aerizusa sponge attached to or grew nearby the scleractinian coral that caused necrosis of the scleractinian polyps. To analyze whether Callyspongia aerizusa sponge released allelochemical that caused the necrosis of scleractinian polyps, 1 liter of seawater in between sponge and coral was taken by syringe equipped with filter paper along with the Callyspongia aerizusa sponge sample. The seawater was extracted with liquid-liquid extraction using ethyl acetate while the sponge sample was extracted with ethyl acetate as well by maceration method. Each extract was dried with vacuum rotary evaporator and analyzed with HPLC (RP column). The result showed that the seawater extract contains four major compounds (at RT 30.52, 35.96, 37.17, and 38.79) that were also found in the sponge extract. This suggests that Callyspongia aerizusa sponge release allelochemicals into sea water to compete and kill the scleractinian coral.