Newly developed serological tests for diagnosing parelaphostrongylosis in cervids, using the excretory-secretory products (ES) of the infective larvae of Parelaphostrongylus tenuis in enzyme-linked immunosorbent assays (ELISAs), have demonstrable superiority over the traditional method of larval recovery and microscopic identification. To generate a source of ELISA antigen by genetic engineering, we created a complementary DNA (cDNA) expression library by the reverse transcription of mRNA of P. tenuis adult worms and by ligation with the vector λ-ZAP II. The library was screened using antisera produced in mice by immunization with a somatic antigen preparation of adult worms. Seventeen clones were isolated, sequenced, and checked for similarity to other DNA sequences in GenBank. A previously identified parasite gene encoding an aspartyl protease inhibitor (API) was isolated from the cDNA library, subcloned, and expressed using the pET expression vector to produce a glutathione S transferase (GST)-His-S-tag-P. tenuis API fusion protein (molecular weight = 63 kDa). An ELISA utilizing the API fusion protein as the coating antigen was used to serologically diagnose all white-tailed deer (WTD, Odocoileus virginianus; 10 out of 10) that had been inoculated with a range of 6–150 L3 P. tenuis, indicating that the antigen may be a useful serodiagnostic antigen for P. tenuis infection in this cervid species.