The origin, differentiation and ultrastructural characteristics of oncospheral envelopes surrounding invasive oncospheres of the dilepidid cestode Dilepis undula are described. In the early preoncospheral phase three primary embryonic envelopes are formed: (1) the capsule; (2) the outer envelope formed by two macromeres; and (3) the inner envelope originating from fusion of two or three mesomeres. Both the outer and inner envelopes of D. undula are therefore cellular in origin and syncytial in nature. Mature eggs of D. undula are slightly oval, measuring 40-50 × 56 um in diameter. Within fully formed eggs, the mature, invasive oncospheres, 36-40 um in diameter, are surrounded by five oncospheral or egg envelopes: (1) outer shell; (2) outer envelope; (3) inner envelope; (4) oncospheral membrane; and (5) hook region membrane covering only one pole of the hexacanth. The ultrastructural characteristics of D. undula oncospheral envelopes are discussed in comparison with those of previously examined dilepidids and other cyclophyllideans.