In organisms that propagate by agametic cloning, the parental body is the reproductive unit and fitness increases with clonal size, so that colonial metazoans, despite lack of experimental data, have been considered potentially immortal. Using asexual propagation rate as a measure of somatic performance, and telomerase activity and relative telomere length as molecular markers of senescence, old (7–12 years) asexual strains of a colonial ascidian, Diplosoma listerianum, were compared with their recent sexually produced progeny. We report for the first time evidence for long-term molecular senescence in asexual lineages of a metazoan, and that only passage between sexual generations provides total rejuvenation permitting indefinite propagation and growth. Thus, this colonial ascidian has not fully escaped ageing. The possibility of somatic replicative senescence also potentially helps to explain why metazoans, with the capacity for asexual propagation through agametic cloning, commonly undergo cycles of sexual reproduction in the wild. J. Exp. Zool. (Mol. Dev. Evol.) 316:276–283, 2011. © 2011 Wiley-Liss, Inc.