Chimpanzees in the Copenhagen Zoo frequently excrete ascarid worms onto the cage floor inspite of a regular anthelmintic treatment program. Previously it had been shown that the source of the infections was of pig origin. However, it was unknown whether the recurrence of the infection was due to reintroduction of eggs from an external source or to a sustained transmission cycle within the Zoo. We found that isolated eggs were able to embryonate into the infective stage and PCR-RFLP analysis on the ITS region amplified from single embryonated eggs suggest these to be Ascaris suum. In addition, sequence analysis of the cox1 gene (‘barcoding') on expelled worms followed by cluster analysis revealed that the chimpanzees are infected with pig A. suum which now, in spite of control efforts, has stabilized into a permanent transmission cycle in the Zoo's chimpanzee troop