Taeniid eggs contain an infective larval form of the parasite, known as the oncosphere, which has been found to be highly susceptible to attack by the host's immune system and this fact has been exploited in the development of highly effective vaccines. Relatively little is known about the structure of taeniid oncospheres and the localization of host-protective antigens within or on the oncosphere. Here, we briefly review the current state of knowledge of the structure of the oncosphere and present preliminary data on the localization of a host-protective antigen within the oncospheres of Taenia ovis. The precise localization of the antigens, in the context of a detailed knowledge of the ultrastructure of the parasite, may reveal the immune mechanisms by which the taeniid parasites are killed by vaccine-induced immune responses, which, in turn, may provide clues about how vaccines could be developed against other parasitic helminths.