AbstractSpermiogenesis inPalaemon serratusis described. The mature sperm has an atypical form and is highly modified. In early stages of spermiogenesis mitochondria, golgi complexes, stacks of annulate lamellae and endoplasmic reticulum are present but disappear later in development. Two modified mitochondria are present in the mature sperm. This latter consists of a cup‐shaped nucleus, a cross‐striated “spike”, has a centriole and lacks an acrosome. The nucleus contains membrane‐bounded vesicles which arise by pinocytosis. The “spike” may be a flexible posterior organ which suggests a mode of fertilization requiring lim