Nanos and nanos-related genes encode RNA-binding proteins and are expressed by stem cells of many metazoans. In the Japanese colonial ascidian Botryllus primigenus, a nanos homologue of 1.3 kb, Bpnos, has been recently identified (Sunanaga et al., 2008), containing two nanos-like CCHC zinc finger motifs. The gene in expressed in germ stem cells, immature and mature male germ cells, as well as in a fraction of circulating haemocytes. Using an anti-Bpnos monoclonal antibody, we confirmed that haemoblasts, i.e., circulating undifferentiated cells, are the only haemocytes recognised by the antibody in both B. primigenus and the Mediterranean species Botryllus schlosseri. In both species, the frequency of haemoblasts reaches the maximum value just after the take-over phase, when the generation change occurs and gradually decreases during the rest of the cycle. The number of circulating haemoblasts is much higher in the Japanese than in the Mediterranean species and this can be related to the occurrence of spontaneous vascular budding in B. primigenus. We also identified a nanos-related gene in B. schlosseri, sharing the nanos domain with two CCHC motifs, which we propose to call Bsnos. We are now trying to get the full cDNA sequence and the deduced amino acid sequence in order to get probes for Bsnos expression studies.