Sperm of marine organisms are usually classified as either “primitive” or “modified”. Primitive sperm, with heads comprised of a simple acrosome, spherical nuclei and mitochondria and a free flagellum, are thought to be associated with the occurrence of external fertilization. The implication is, of course, that external fertilization is a primitive condition wherever it occurs. Here the sperm of two Scyphozoa,Catostylus mosaicus(Quoy and Gaimard 1824) andPhyllorhiza punctatavon Ledenfeld 1884, are described. Both of these taxa are known to brood planulae and are likely have some sort of internal fertilization.Catostylus mosaicushas a typical “primitive” sperm with a small head (2μm long) comprised of a short cylindrical nucleus, four spherical mitochondria and a some small spherical electron dense spheres that are may be acrosomal in function. The anchoring apparatus is comprised of two centrioles, a complex of satellite fibres and a striated “spur”. The flagellum contains a typical 9+2 axoneme.Phyllorhiza punctatahas sperm with a head 6 μm long. The nucleus is a narrow cylinder, 4 μm long and curved in one plane. There are some anterior electron dense spheres, and five ellipsoidal mitochondria abut the nucleus posteriorly. Behind the mitochondria an anchoring apparatus comprised of the distal centriole and a satellite array is connected to the bases of the mitochondria. The flagellum contains a typical 9+2 axoneme. Sperm structure and the various reproductive modes occurring in Scyphozoa are reviewed and placed in a phylogenetic context based on the current classification of the group. It is concluded that external fertilization is likely to be the plesiomorphic condition in Scyphozoa, but there are various possible scenarios about the transformation to and from external fertilization within the group. Sperm structure is shown to be a poor predictor of reproductive mode. It is recommended that the terms “primitive sperm” and “modified sperm” be abandoned and replaced with terms that describe sperm in terms of function.