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The structure of the sperm of Aplysia californica was studied by both transmission and scanning electron microscopy.  Aplysia californica, a species with internal fertilization, has the modified type of molluscan sperm structure.  Spermatids had a glycogen helix spiraled about the flagellum, both enclosed by a common microtubular basket.  A second vacuole helix was periodically seen only in spermatids and absent in spermatozoa.  An additional basket of microtubules appeared to direct the elongation and spiraling of the nucleus about the flagellum/glycogen helix. A flat acrosome was present while the centriolar derivative was embedded in a deep nuclear fossa with strands of heterochromatin arranged nearly perpendicular to its long axis.  The mitochondrial derivative consisted of small, frequently electron dense, closely spaced rods but individual mitochondria were also seen surrounding the axoneme of spermatids.  The axoneme consisted of dense fibers that appeared to have a "C" shape substructure with a central dense fiber thus providing a 9+1 arrangement of singlet units; the typical 9+2 microtubule arrangement of flagella was absent. Flagella with two axonemes were frequently seen as well as an extra axoneme within the head of immature sperm.