The 45W vaccine antigen of the cestode parasite Taenia ovis is a member of a small gene family estimated to comprise six members. All six genes were cloned and characterised. Nucleotide sequence analysis of the different family members identified a set of closely related genes exhibiting between 75.2 and 98.6% DNA sequence homology. Intron/exon structure for the various genes constituting the family was found to be conserved. Open reading frame analysis and pairwise alignments of predicted polypeptides from the various members of the family revealed that the encoded proteins share between 51.8 and 96.5% amino acid identity with the host-protective 45W antigen. Polymerase chain reaction (PCR) primers were designed which allowed amplification of individual family members in a transcript-specific manner. These were used to investigate gene expression in different developmental stages of the parasite by reverse-transcription polymerase chain reaction (RT-PCR). All members of the gene family were expressed during the T. ovis life-cycle. 45W gene family transcription correlates with embryogenesis of eggs and expression is increased in activated eggs (oncospheres). One family member, ToW3S, was expressed in the early larvae (cysticercus) stages. Expression of genes closely related to the 45W vaccine antigen has important implications for immune evasion under selection from the application of a 45W-based vaccine.