A polymorphic set of 14 kDa excretory–secretory (E–S) antigen-encoding cDNAs, with similarity to a previously characterized 15 kDa E–S antigen of Haemonchus contortus, was cloned from Cooperia punctata. Five cDNAs encoding predicted proteins of 70–80% identity were sequenced. Nucleotide sequence data reported in this manuscript are available in the GenBankTM databases under the accession numbers AF352718, AF352719, AF352720, AF352721, AF352722, AF352723, AF352724, AF352724, AF352726 and AF352727. Genomic analyses of individuals proved the existence of three 14 kDa E–S antigen-encoding genes, excluding that the differences reflected polymorphisms between individuals in a population. Southern blots indicated the presence of additional members of this gene family. Thus, despite the fact that heterologously expressed C. punctata 14 kDa E–S products are shown to be recognized by immune sera, potential pitfalls in the development of a recombinant vaccine are presented by this genetic diversity. Vaccine design could be further rationalized by knowledge of the function, and possible redundancy in function, of the E–S products which is presently lacking. The limitations encountered in assigning a function to the 14/15 kDa family of E–S proteins that is thus far unique to the trichostrongyloid nematodes are discussed.