A study was made of the vertical distribution, diel migration and concentration al the sea surface of pelagic Peracarida (Mysidacea, Amphipoda), in the axis of two submarine canyons, over the French continental slope. Two stations were worked, one centred at 42 degrees 28.50N, 5 degrees 59.40E, in the Toulon canyon (Northwestern Mediterranean) over bottom soundings ranging from 1100 to 1400 m, and another centred at 44 degrees 43N, 2 degrees 18E, in the Cap-Ferret canyon (Gulf of Biscaye, Atlantic coast), over bottom soundings ranging from 2100 to 2400 m. Series of depth-stratified opening-closing OMORI net tows were successively achieved during several diurnal cycles. Nets are operated acoustically. Eight depth ranges were sampled with oblique hauls, from 100 to 400 m above the bottom lo the surface. Hyponeustonic species were identified from replicated neuston net hauls. Eleven species of Mysidacea and 19 species of Amphipoda were recorded. Four Mysidacea and 10 hyperiid Amphipods were sufficiently abundant to allow a description of the vertical distribution and diurnal migration. The Mysid Encopia unguiculata was the most abundant bathypelagic species in the two sampling areas: the maximum concentration was 150 specimens per 2000 m(3) in the Mediterranean and 93 specimens per 2000 m(3) in the Atlantic. Adult and juvenile Mysids are concentrated in the 600-1600 m depth range in the Atlantic; a little diel vertical migration, occurring in the upper 600 m only, is observed in juveniles. In the Mediterranean Sea, a highest extent of vertical migration is observed: animals, concentrated during the day at 1000-1200 m depth range, near the bottom layer, migrated at night up to 400 m. The only meso-epipelagic Mysid, Siriella thompsoni, exhibits a clear nocturnal hyponeustonic behaviour in the Mediterranean Sea: a stratification in the neuston is observed from dusk to dawn. During the day, animals dispersed in mesopelagic water. Amphipod fauna included a bathypelagic gammarid, Cyphocaris anonyx, occurring in the 200-1000 m hauls in the Atlantic, and bathy, infra, meso and epilelagic hyperiids. Ten species exhibited a clear nocturnal hyponeustonic behaviour: adult males only (Eurpronoe minuta, Lyncaea pulex, Platyscelus serratulus, Anchylomera blossevillei) or males, females and juveniles (Lestrigonus schizogeneios, Hyperietta stephenseni, Parathemisto gaudichaudii) ascended to neuston at night. The behaviour (hours and durations of the stratification) was specific for each species. In the Atlantic, P. gaudichaudii was the only hyperiid occurring abundantly in the neuston: stratification in the neuston started at dusk and ended at dawn; during the day, the animals descended down to 500 m. Over the margins, these stratifications in the neuston can induce, by onshore wind regimes, a nocturnal superficial advection of infra and mesopelagic species of Peracarida towards the slope edge and the continental shelf.
