Total DNAs, isolated from a range of taeniid cestodes (Taenia solium, T. saginata, T. pisiformis, T. crassiceps, T. hydatigena, T. ovis, T. multiceps and T. taeniaeformis), have been subjected to restriction enzyme digestion, Southern transfer and hybridization analysis using cloned fragments of the ribosomal RNA gene of Schistosoma mansoni. Substantial inter-specific genetic differences have been revealed on the basis of characteristic hybridization patterns for each of the taeniid cestode species. Furthermore, a random genomic DNA library has been constructed in the vector plasmid pAT153 using DNA extracted from a pig isolate (Indian origin) of T. solium. A panel of taeniid cestode DNAs including DNA from Echinococcus granulosus, has been used in conjunction with hybridization and restriction enzyme analysis to identify in the library a single recombinant plasmid with a T. solium-specific insert (coded pTS10) and two recombinant plasmids with T. solium inserts having selective specificities for T. solium and T. ovis (coded pTS17) and T. solium, T. saginata, T. ovis and T. multiceps (coded pTS28). These recombinant plasmids and the cloned fragments of the ribosomal RNA gene of S. mansoni have been used in restriction endonuclease, Southern transfer and hybridization analysis to detect intra-specific genetic variation in cysticerci of T. solium from India, Mexico and Zimbabwe. In addition, pTS10 and pTS17 have been used in a simple dot-blot assay to distinguish T. solium from T. saginata.