Ovisac secretion by the paired colleteric glands of Sacculina carcini and Heterosaccus dollfusi (Rhizocephala, Sacculinida) was documented and studied at the ultrastructural level. Preparatory to oviposition, the epithelium of each colleteric gland secretes one branched, elastic, transparent ovisac. The ovisac wall consists of a reticulated inner zone, secreted first, and a dense outer zone. After secretion, the ovisac detaches from most of the secretory epithelium but remains anchored proximally in the gland until oviposition ends. The exterior ovisac surface is predominantly smooth and impervious. Proximally, however, the surface is irregular and perforated. During oviposition the eggs enter the paired ovisacs, forcing the ovisacs through the ovipores into the maternal mantle cavity. Simultaneously the ovisac volume increases approximately 100 times. The resulting paired egg masses, branched like the ovisacs, are brooded and ventilated in the mantle cavity. Ovisacs prevent that developing embryos are lost prematurely with the ventilation current. Within two days the egg masses solidify and attach to retinacula in the mantle cavity cuticle. The ovisacs, now probably obsolete, are no longer discernible. The literature on colleteric gland morphology implies that ovisacs are secreted by all Sacculinidae and perhaps other Rhizocephala with colleteric glands. Similarities in both the secretory process and morphology suggest homology between the colleteric glands of Sacculinidae and the oviducal glands of Thoracica (Cirripedia) and between their products.