Problems concerning the structure of shell and manner of its growth in the genus Creusia Leach, 1817, which have been examined on the example of a Miocene species, Creusia sanctacrucensis n.sp., constitute the subject of the present paper. All Miocene representatives of this genus, described so far, have been revised. It has been shown that Creusia Leach, 1817 and Pyrgoma Leach, 1817 are two different genera which, together with Pyrgomina Bałuk & Radwański, 1967, make up - within the family Balanidae Leach, 1817 - a separate subfamily, Creusiinae n. subfam. Furthemore, problems related to the ecology of the Creusiinae have been examined. It has been shown the growth of shells of these cirripeds is possible only within live and still growing anthozoans. The mutual relation of cirripeds and anthozoans should be determined as a commensalism. The stratigraphic distribution of the Creusiinae and problems of the phylogeny of this subfamily have subsequently been examined. On the basis of comparison of the shell structure in the genus Creusia Leach and particular ontogenetic stages in the genus Balanus da Costa, the conclusion has been drawn that the Creusiinae separated from the main stock of the Balanidae as a result of the neotenic development of a certain branch of this family which probably took place in the Oligocene.