Reconstruction of the phylogenetic history of a parasitic group is clearly difficult due to a lack of fossil forms. Among the nematodes parasites of vertebrates, the superfamily Trichostrongyloidea is one of the richest groups in terms of number of species (more than 1000 described) and genera (175) and they therefore offer excellent opportunities to reconstruct their phylogeny. Trichostrongyles occur in the gut and less commonly in the stomach and the lungs of all classes of terrestrial vertebrates except the Perissodactyls and the Proboscidian. They have a world wide distribution and direct life cycle. The criteria used in the classification of the Trichostrongyloidea are essentially morphological. Morphological characters are numerous and an attempt has been made to distinguish their relative value. Information concerning the synlophe, which is the apparatus of locomotion and attachment of the worms in the gut of the hosts, has been found essential in order to construct a classification of the superfamily and understand its evolution. This analysis was greatly facilitated by information from two sources: 1) the morphology of free-living rhaditids, which are the ancestors of the trichostrongyles; 2) the ontogeny of the synlophe. The classification established above takes into account the different morphological characters, but only concerns extant animals. Data provided by the hosts permit us to date the appearance of different lineages and to follow their evolution in time and space. These data come from extant hosts and from paleobiogeography. 1) The evidence from extant host species includes the host distribution of the trichostrongyles in relation to the geographic distribution of the hosts. 2) The evidence from extinct forms includes the date of appearance of the host in the geological record as well as mammalian migration in geologic time periods: a) The date of the appearance of the hosts in the geological records permits us to date the origin of the different families of parasites. b) The manner in which host migration interacts with the parasite is very complex and a few examples will be considered, mainly from the caviomorphs, the sciuromorphs and the myomorphs. The combination of evidence from the morphology of the worms and evidence from the paleobiogeography of the hosts allows us not only to explain the present day host and geographic distributions of the parasites but also to reconstruct their evolutionary history. The phylogenetic tree of the Trichostrongyloidea proposed with Chabaud consists of three main branches which are morphologically clearly defined.(ABSTRACT TRUNCATED AT 400 WORDS)