P53 gene family members (p53, p63 and p73) are involved in crucial cellular functions such as cell-cycle arrest, apoptosis and development. In this study four partial p53 family cDNAs (cDNA1-4) and the first complete genomic sequence of the p53 family homologue in the mollusc Mediterranean mussel Mytilus galloprovincialis were identified and characterized. Results indicate gene structure of M.galloprovincialis p53 family homologue to be most similar to vertebrate p63 gene, considering p63 as the ancestral gene of the p53 family as well as the only gene of the family present in non-chordate metazoan species. The M.galoprovincialis p63 gene spans through 20620 base pairs (bp) and is composed of 15 egzons interrupted with 14 introns. Analysis showed the new M.galloprovincialis cDNA (cDNA1-4) sequences code for TAp63 or TAp63 isoforms, and identified all other molluscan p53 family sequences as p63 gene or its expression isoforms. Comparison of M.galloprovincialis p63 gene structure with other orthologues, found in sequenced genomes of four molluscs (Aplysia californica, Lottia gigantea, Crassostrea gigas and Biomphalaria glabrata), revealed the conserved p63 gene structure in the phylum Mollusca. Analyses also detected possible mis-annotations of the p63 gene in sequenced molluscan genomes and stressed the value of manual inspection (based on alignments of cDNA and protein onto the genome sequence) for a reliable and complete gene annotation.