SUMMARYThe transforming growth factor-β (TGF-β) gene family regulates critical processes in animal development, and plays a crucial role in regulating the mammalian immune response. We aimed to identify TGF-β homologues from 2 laboratory model nematodes (Heligmosomoides polygyrusandNippostrongylus brasiliensis) and 2 major parasites of ruminant livestock (Haemonchus contortusandTeladorsagia circumcincta). Parasite cDNA was used as a template for gene-specific PCR and RACE. Homologues of the TGH-2 subfamily were isolated, and found to differ in length (301, 152, 349 and 305 amino acids respectively), with variably truncated N-terminal pre-proteins. All contained conserved C-terminal active domains (>85% identical over 115 amino acids) containing 9 cysteine residues, as inC. elegansDAF-7,Brugia malayiTGH-2 and mammalian TGF-β. Surprisingly, only theH. contortushomologue retained a conventional signal sequence, absent from shorter proteins of other species. RT-PCR assays of transcription showed that inH. contortusandN. brasiliensisexpression was maximal in the infective larval stage, and very low in adult worms. In contrast, inH. polygyrusandT. circumcincta, tgh-2transcription is higher in adults than infective larvae. The molecular evolution of this gene family in parasitic nematodes has diversified the pre-protein and life-cycle expression patterns of TGF-β homologues while conserving the structure of the active domain.