During 1½ year the biology of Corallonoxia longicauda, a copepod endoparasitic in the stony coral Meandrina meandrites was studied in Curaçao, Netherlands Antilles. The infestation rate of the corals as well as the numbers of parasites present were investigated at several depths and in several stations. The parasites proved to be distributed in a spatial pattern inside the colony. This pattern appeared to be correlated with current and exposure. It is postulated that the numerical distribution of the copepods in the corals, which differs markedly from a normal distribution, depends on the way of proliferation and settling of the parasites. This offers also an explanation for the remarkable spatial pattern inside the colony. The results of the comparison of the biomasses of host and parasite indicate that the parasite must be rather harmless to its host, and that no significant influence of its metabolism on the skeleton formation of the coral is to be expected.