Fresh specimens were collected from natural infections and from experimentally infected animals. Collections were made in the United Kingdom and in Nigeria and a few specimens were also obtained from other countries abroad. Materials from the specimens was compared by staining, serological techniques and enzyme electrophoresis. Staining ova by the Ziehl-Neeleen method was not successful in differentiating ova of any of the species including T. saginatue and T. solium. Serological techniques that were tried included oval precipitation, agar gel diffusion precipitation, fluorescent antibody, immunoeleotrophoresis and the enzyme-linked immunosorbent assay (ELISA). Strobilate, oval and, in some instances, larval cyst antigens, were used in tests against antisera raised in rabbits and other species. With antisera that had been absorbed by one or two heterologoue antigens, some success was achieved in agar gel diffusion precipitation tests and with ELISA in differentiating strobilate tissue, but serological techniques were not generally adequate for this purpose. Success in differentiating taeniid material was achieved by enzyme electrophoresis on thin layer starch gels. Of the many enzymes tested, five were found to display inter-species zymogram differences in the pattern and mobility of their multiple forms: adenylate kinase, glutamate dehydrogenase, malate dehydrogenase, hexokinase and glucose phosphate isomerase, of which the latter two were the most useful for differentiation. No differences were seen between larval and adult material except for the presence of host enzymes in the former which were easily identified by comparison with host tissue extracts. No differences were seen between immature, mature and gravid proglottides and no individual variation was seen between worm specimens. Anthelmintic treatment did not affect worm enzyme patterns. Taeniid species were compared with one another and also with other tapeworm species. The mobility and pattern of the enzyme forms in zymograms differed more between unrelated than between related species. With glucose phosphate isomerase it was possible to identify most of the taeniid species. When this enzyme was preserved in a solution containing enzyme stabilisers, it stored well over a long period especially in liquid nitrogen or, freeze dried and kept at -20 °C. Dilution of samples did not distort zymogram patterns and crude extracts could be identified using the mobility of the slowest moving band as the criterion. This method was used successfully for a survey in Nigeria. Identification was by comparison with a known species. The other enzymes were useful for confirming the diagnosis. Strain variation between E. granulosus of horse and sheep origin was seen in specimens collected from Belgium and different parts of the United Kingdom. Cysts from oxen were identical to those from sheep, but indirect comparison with cysts from Nigerian camels showed these to be different from both the equine and ovine strains.