Unipolar ingression is a common form of gastrulation in the phylum Cnidaria. Here, this process is examined in the hydrozoan Phialidium. Ultrastructural investigation confirmed that, during this process, oral cells elongate, constricting at their apical ends and/or expanding at their basal ends as they ingress. Marking studies show that most entodermal cells originate from the oral area, but a few also come from lateral, aboral-lateral, and aboral areas. Based on orientation of mitotic spindles during gastrulation, a few cells may also enter the entodermal region by delamination. Cell sorting experiments showed that, by postgastrulation, cells from aboral areas of the preingression embryo developed lower adhesive capacities than those from oral regions. Differences in adhesive capacity may allow maintenance of distinct cell layers. Postgastrulae are shorter, thinner, and more streamlined than preingression embryos. Total embryo volume and cell volume decrease, the blastocoel disappears, and 34.4% of the cell mass becomes entoderm. Activities of the ectodermal layer play an important role in gastrulation: during gastrulation, aboral cells become more columnar and oral cells less columnar. Also, cells in oral and oral-lateral regions shift toward the midline, causing oral portions of the embryo to elongate. A model of unipolar ingression is proposed.