Protoscolices ofEchinococcus granulosus isolated from hydatid cysts of sheep and donkeys in Jordan were cultured in vitro using a modified diphasic culture system. Protoscolices from these two sources manifested differences in the mode of development, evagination and growth rates. Protoscolices isolated from sheep cysts grew in vitro in the polyzoic direction up to the three- to four-segmented mature worms, reaching a length of 2.9 mm. In contrast, donkey protoscolices failed to develop beyond the early stages, even after 67 days of culturing. On prolonged culturing, few worms of donkey origin reached the banding and segmentation stages, attaining a maximal length of 1.6 mm at periods ranging between 81 and 114 days of culturing. None of these segmented worms showed genital differentiation or analgen. The evagination and growth rates of protoscolices isolated from donkey liver cysts were compared with those obtained from sheep liver or lung cysts. The most significant difference in these rates occurred at the commencement of the segmentation stage. Differences in the development, growth and evagination rates observed between the donkey and sheep forms may reflect the strain variation ofE. granulosus in this country.