Pylochelid Pagurids differ mostly from all other members of the section by a well developped abdomen, in which all segments are articulated and provided with a pair of appendages, similar in this way to many other Reptant Decapods. They are commonly called " symmetrical " Pagurids, but this is not correct, since in one genus the abdomen, telson and pleopods are noticeably asymmetrical. Our knowledge of the group was restricted to 16 species, recorded from a few rather deep water stations in Indo-West-Pacific and Western Atlantic, most of them known only from their type localities. The abundance of new material, originating mainly from Albatross dredgings and from recent French explorations in the I.W.P. has led to the present systematic revision. As a result, 24 new species or subspecies are added to the 16 previously established valid species ; the five known genera, Pomatocheles, Pylocheles, Mixtopagurus, Cheiroplatea, and Parapylocheles, have been redefined, some species of Cheiroplatea transfered to Pylocheles and the latter divided into three subgenera (Pylocheles, Xylocheles subgen. nov. and Bathycheles subgen. nov.). Besides, two genera, Cancellocheles gen. nov. and Trizocheles gen. nov. are created. The Pylochelidae could be considered up to now as a restricted family of infrequent species : apart from 3 forms reported in several occasions from Japanese waters, the whole number of specimens recorded in literature did not exceed 60, captured in about 30 stations. The present revision includes more than 400 specimens, collected in ca. 200 stations ! The importance of Pylochelid fauna in tropical and subtropical waters must therefore not be neglected, and, most probably, new taxa and new localities will be added in the future. This research however has not been restricted to the description of new forms. Investigations on relationships between the various genera have shown that the whole group is made up of several distinct phyletic lines, whose respective affinities do not appear clearly, and the family had to be divided, at least provisionnaly, into 6 subfamilies. Regarding the systematic position of the Pylochelidae within the section Paguridea, they are classified in the superfamily Coenobitoidea, and a comparative study of their main characters suggests that they are close to the family Diogenidae. They cannot however be regarded as primitive representatives of that family : both Diogenidae and Pylochelidae probably have a common ancestor, but evolved separately along various phyletic lines. In the taxonomic part of this work is also described and illustrated for the first time the glaucothoe stage of a Pylochelid, Pomatocheles stridulans sp. nov. The richness of the new material at the origin of the systematic revision of the family has also provided a quantity of information on the ecology or the habitat of many forms, and on the interpretation of various adaptive morphological structures. According to their dwelling, genera and subgenera can be classified, as a whole, as xylicolous, petricolous, tusk-dwellers, spongicolous, with a few specific or individual exceptions. In connection with the habitat, adaptive features have been described : opercular structures, boring " r a s p " , stridulating apparatus... The Pylochelidae are known from two disjunct areas, the Indo West-Pacific (36 species or subspecies in 6 genera and 5 subfamilies) and the North Western Atlantic (4 species in 3 genera and 2 subfamilies). In Indo- West Pacific, their distribution is extremely wide, from South Africa to the Kermadec Islands, and from Japan (ca. 38° N) to southern New Zealand (ca. 46° S). Indonesia, with 14 species and 5 genera appears as the center of dispersion and diversification. Japanese endemism is noteworthy : one genera and 6 out of the 7 species have not been reported elsewhere. In North Western Atlantic Pylochelidae, poorly represented, extend from Bardados to the North Western part of the Gulf of Mexico and from ca. 10° N to 35° N. Two genera only, belonging to the sole non monotypic subfamily (Pylochelinae) provide a biogeographical link, probably from Tethyan origin, between the two areas. The probable relation between the availability of dwelling material and the geographical distribution is also discussed. The vertical distribution extends from 30 to 1,570 meters, but the group is mostly represented between 200 and 500 m, where 28 species have been found. 3 species only are presumably usually living above 200 m, 9 have been recorded from 500 to 750 m and no more than 5 beyond. An english synopsis of subfamilies, genera and subgenera of Pylochelidae is given in an appendix (p. 250).