The results of the chemoprophylaxis of experimental hydatidosis with a combination of praziquantel and albendazole are presented. In vivo studies were performed on NMRI mice injected with protoscolices of Echinococcus granulosus. The treatment was the same as that previously used by us , but reducing the treatment period to only 1 month. Four months after injection , the mice were killed and autopsied. Developed hydatid cysts were counted, weighed and processed for observation by transmission electron microscopy. The efficacy of the present treatment was 97.7% and 97.15%, in number and weight of the cysts respectively, only slightly lower than that obtained by us after 4 months of treatment. Moreover, the few cysts that had developed in the treated mice showed severe damage to their ultrastructure.