The ultrastructural aspects of some cellular types in Aelosoma have been investigated. Aelosomatidae are aquatic Oligochaetes in which the asexual reproduction isvery common, while sexual individuals are rare or, in some species, have never been observed. Asexual reproduction occurs by fission and regenerative processes precede the separation of the daughter zooid. In this work are described the ectodermal cells of the parent's body (including gland cells) and the blastematic cells of the daughter zooid. Between these blastematic cells occur some nerve fibres, endowed with neurosecretory granules. The neurosecretion seems then to activate the asexual reproduction.