Synopsis.A new species of Microsporida was observed in neurons of the marine molluscAplysia californicaand the ultrastructure of its spores was investigated. The spores were ˜ 1.3 m̈m long and had a thick 3‐layered coat. An anchoring disc and polar aperture were present. The polaroplast occupied most of the anterior half of the spore. The vesicular portion included lengths of loosely packed lamellar structures as well as the usual vesicular profiles embedded in fibrillar material; it was entirely surrounded by tightly packed, electron‐dense lamellae. The polar tube had 5 or 6 coils. In this stage of the life cycle (mature spores), the parasites did not appear to be disturbing the host cells. The organism was namedMicrosporidium aplysiae