A full-length cDNA encoding a highly conserved calmodulin was isolated from a cDNA library prepared from hatched larvae of the ascidian Ciona intestinalis. Sequence analysis has identified a 447 b.p. open reading frame, encoding a putative protein of 149 amino acid residues, with a predicted molecular weight of 16.8 kDa, showing 85-98% identity to known calmodulins. Northern blot analysis revealed a single transcript of about 0.8 kb in length, which was maternally expressed and progressively increased during development, until late tail-bud stage. Whole-mount in situ hybridizations, carried out on embryos at different stages of development, showed that starting from the neurula stage, the C. intestinalis calmodulin (Ci-CaM) expression became restricted to the neuroectoderm and that in larvae it was specifically detected in the nervous system.