Eimeria are the causative agents of coccidosis, a disease which is of world wide economic importance in the poultry industry. Immunity resulting from infection is species-specific and both antibody and cell-mediated responses have been implicated. As an initial step in the development of a genetically-engineered vaccine against coccidiosis, libraries of EcoRI-digested genomic DNA from E. tenella have been constructed in Escherichia coli using the expression vector lambda amp3. Screening of the libraries with serum from chickens immunized by infection has identified at least 24 different recombinant phage which produce eimeria antigens fused to beta-galactosidase. A significant proportion of the Eimeria DNA inserts cross-hybridise with each other and contain sequences which are highly represented in the genome. The identification of these clones will enable the isolation of intact genes from E. tenella DNA and facilitate detailed analysis of the antigens and immune responses.