Abstract. The minimum effective concentration of praziquantel against protoscoleces of Echinococcus granulosus (ovine strain) was determined in vitro; 20 μg/litre gave statistically significant results. No difference was seen between the sensitivity of ovine and equine protoscoleces at 50 μg/litre. Morphological observations on treated protoscoleces and passage into gerbils both suggested that the eosin-exclusion technique overestimates viability. Electron microscopy of treated protoscoleces showed severe, disruptive tegumentary damage. Praziquantel is thus an extremely active protoscolicidal agent that may have an important peri-operative role in preventing recurrence.