The complement system represents an important humoral component of the mammalian immune system. Complement components can be subdivided in 5 gene families: C3/C4/C5, Bf/C2, MASP/C1r-s, C6/C7/C8A/C8B/C9 and Factor I. Until 1884, it was generally believed that the complement system was an unique feature of vertebrates since all attempts to identify complement components in invertebrates failed. In recent years, the genomic approach revealed the presence of complement orthologue genes in invertebrate deuterostomes, mainly in sea urchins and tunicates (ascidians). Conversely, no complement genes were found in the genome of protostomes such as Drosophila melanogaster and Caenorhabditis elegans, suggesting that the complement system was established in the deuterostome lineage. Genome analyses carried out in the solitary ascidian Ciona intestinalis revealed that most complement gene families are present in urochordates. We recently carried out the assembling of EST collections from the colonial ascidian B. schlosseri, obtained in our and other laboratories: we found multiple transcripts showing high similarity with vertebrate complement components such as C3, MASP, MBL and C6. Preliminary in silico studies revealed close relationships between Botryllus C3, MASP and MBL and orthologues from other chordates. In particular, C6 seems related with the and Halocynthia roretzi and share with them the absence of the FIM domain which is responsible for the interaction with the other complement molecules in vertebrates. Future studies will be devoted to the analysis of the expression of genes for complement components of B. schlosseri.