We have used nuclear (ribosomal ITS1) and mitochondrial (ND1) sequences to characterize human and pig isolates of Echinococcus granulosus collected by fine needle aspiration biopsy (FNAB) in Poland. The data indicate clearly that the Polish patients were not infected with the common sheep strain (G1 genotype) of E. granulosus, normally associated with human cystic hydatid infection. Instead, the hydatid parasite infecting the Polish patients shares very similar ND1 sequence with the previously characterized pig (G7) genotype but it also exhibits some clear differences. In particular, E. granulosus DNA from the Polish patients amplified a single ITS1 fragment in PCR and distinct ITS1-RFLP patterns were obtained after restriction digestion. The form of hydatid isolated from the Polish patients appears, therefore, to represent a distinct, previously undescribed genotype (designated G9) of E. granulosus.