The formation of aggregate colonies in Eudendrium glomeratum is described from the rocky cliff of the Portofino Promontory, Italy. Two distinct mechanisms may give rise to this phenomenon: (i) aggregation and growth together of planulae, (ii) settling of planulae on adult colonies. Generally the aggregate colonies are gonochoric but two cases of colonies showing males and females branches are described. Aggregation of the planulae and their settlement on other colonies is likely to be regulated by histocompatibility, as happens in Hydractinia echinata.