Early shell formation was examined in embryos of the opisthobranch gastropod Aeolidia papillosa. Secretion of the first organic shell material occurs prior to closure of the shell gland lumen, contrary to reports for other molluscan embryos. This difference suggests that in externally shelled gastropods and bivalves initiation of shell secretion may be coincident with narrowing of the shell gland pore rather than with closure of the lumen. The shell growth region was examined ultrastructurally. As no shell material is seen in the shell gland lumen, the shell gland seems not to be actively involved in shell secretion. Initial shell material is secreted only by cells surrounding the shell gland pore. Shell material seems to be added, not in a gap between cells as previously described, but over the apical surface of cells at the growing edge. The growing edge of the shell and the growing edge cells are covered by cytoplasmic extensions arising from the neighboring cells distal to the shell gland. No infoldings of the growing edge cell membranes are seen. The first organic shell material is 20 nm thick, consists of two electron dense layers separated by an electron lucent layer, and is secreted at least 33 hours (5°C) before shell mineralization, as detected by polarizing microscopy.