The  present  study  was  conducted  to  determine  viability  rate  of

   protoscoleces and fertility rate of hydatid cysts isolated  from 118 sheeps

   and 114 cattles slaughtered at Zanjan abattoir during 1998.

         At  first ,  cyst  number  of the infected organs was counted and their

   diameter was measured.  Then  by  dissecting the cysts , their fertility was

   determined.  To  determine   protoscoleces   viability  ,   eosin  -  staining

   technique   was applied.

         The results indicate, in infected sheep, cysts were found in 17% of the

   livers , 47.5% of  the  lungs and 35.5% of the livers and lungs concurrently,

   with  cyst  fertility  rate  of  49.15%  and  protoscoleces  vialility  of  67%. In

   infected cattle cysts  were found in 5.26% of the livers, 80.7% of the lungs

   and  14.04%  of  the  livers  and  lungs  concurrently , with  protoscoleces

   viability of 58%. 96.5%  of  the  cattle  organs  were  found  infected  with

   mixed fertile and  infertile  cysts.  Frequency  of  the multivesicular cyst in

   both liver and lung was higher than univesicular cyst. Fertility rate of liver

   cyst  was  higher  than  lung  cyst  and  multivescular  cyst   higher   than

   univesicular cyst. No significant difference between protoscolex viability

   rate of  liver  and  lung  as  well  as multivesicular  and univesicular cysts

   were  observed.