This study provides information about the reproductive cycle of the invasive side-gilled sea slug Pleurobranchaea maculata, currently widely distributed along the Argentine coast. The histological analysis of this simultaneous hermaphrodite showed characteristics of a semelparous species with an annual life cycle. The highest spermatogenic and oogenic activity was observed from August to January. Partial evacuations of mature oocytes, measuring over 100 mu m in average diameter, occurred continuously during the long spawning season. Oviposition events started in winter and increased in frequency with increasing photoperiod (hours of light) until summer solstices. During February and March, postreproductive and immature individuals coexisted, while mature individuals were rarely observed. These results represent important information on the reproductive patterns of an invasive species of worldwide ecological and sanitary concern.