Four species of Ordovician Chitinozoa from Öland, Sweden, Cyathochitina stentor, Desmochitina minor, Lagenochitina esthonica, and Lagenochitina tumida, have been investigated by transmission electron microscopy. There is very great variation in the vesicle wall ultrastructure of the specimens studied. The texture of the vesicle wall can be homogeneous or granular and can also contain denser zones. Ultrastructures such as pore canals and ‘empty spaces' have been revealed. The systematic affinity of the Chitinozoa is discussed, and it is concluded that chitinozoans display typical metazoan structures represented in eggs and egg capsules of marine invertebrates and possibly also in fish.