ABSTRACT. Worm burdens were evaluated and compared in two groups ofthe guinea pig, Cavia poreellus (Linnaeus, 1758): animais of the first group were conventionally maintained in an institutional animal house and those of the second group were openly kept in pet shops in Brazil . Animais from both sources were infected on ly with the nematode Paraspidodera llncinata (Rudolphi, 1819) Travassos, 1914 (10% of prevalence in guinea pigs from the institutional facility and 40% in those animaIs from the pet shop). Other helminth samples recovered from Brazilian guinea pigs during 52 years and that are deposited in the Helminthological Collection of the Oswaldo Cruz Institute (CHIOC) were also analyzed. Paraspidodera llneinala and the cestode Monoecoceslus parcilesliculalus Rêgo, 1960 were identified in these samples. KEY WORDS. Helminths, Paraspidodera uncinala, Monoeeoeeslus parcilesliculalus, Cavia poreel/us, guinea pig, Brazil Taking into account the concern laboratory and, most recently, pet animais arise worldwide and based on an established pragram (present authors) scheduled to control the sanitary conditions of different animal houses that supply Brazilian institutions with laboratory animais for scientific research, investigations have been developed in order to provide a reliable survey of the helminth fa una occurring in several animal models, mainly radents, commonly maintained, but sei dom evaluated, considering the few data avail able regarding the parasites they may harbor. The proper identification of the helminths infecting laboratory animais is necessary, considering that parasites may act as variables, usually unsuspected , in experiments.Helminths ofmice, rats and hamsters from Brazilian animal facilities have been investigated and the present data add new information to previous reports of the helminth fauna of laboratory and pet rodents in Brazil (PINTO et aI. 1994, 2001a, b, GONÇALVES etal. 1998.The present survey is related to nematode parasites of either conventionally maintained or openly kept guinea pigs. An extra amount ofhelminth (nematode and cestode) samples previously recovered fram these hosts in Brazil were also identified and included.