A small, size selected (0.5-5.0 Kbp) genomic DNA library has been constructed in the bacterial plasmid pAT153 using DNA extracted from a human isolate (Kenyan origin) of the hydatid disease organism, Echinococcus granulosus. A panel of taeniid cestode DNAs has been used in conjunction with hybridization and restriction-enzyme analysis to identify in the library two recombinant plasmids with Echinococcus-specific inserts and a single recombinant plasmid (coded pEG18) with a DNA fragment unique for E. granulosus. These, and other recombinant plasmids with E. granulosus DNA inserts, have been used in restriction endonuclease, Southern transfer and hybridization analysis to independently and reproducibly discriminate between the UK horse and sheep strains of E. granulosus; these probes may well prove of value for characterizing isolates from other endemic areas. The feasibility of using a cloned DNA fragment as the basis of a simple field test for distinguishing eggs of E. granulosus from those of other taeniid cestodes is discussed. The recombinant insert (approximately 2.3 Kbp in size) of pEG18 has a low copy number - estimated at approximately 26 - and it may not be sufficiently sensitive for practical use as a DNA probe in the identification of small numbers of E. granulosus eggs by molecular hybridization.