Two phreatic stations providing old meanders of the French river Rhône (“Lônes”) with interstitial water have been studied for three years. The samples (100 liters of water) have been collected by the Bou-Rouch method at 60 cm deep in the sediment. The analyses of populations show that the biocenoses of the two stations are quite different: The one (Station 2) is rather specialized; most of the species are troglobitic (80% of the whole biomass; the only Amphipod, Niphargopsis casparyi, represents 67% of this biomass). The other (Station 8) is very diversified; the seven dominant groups are all epigean animals. Population numbers have changed during the three years of sampling. Fluctuations have been observed in station 2, but the total numbers were quite similar in 1975 and 1977. On the contrary, station 8 shows an “exponential” type of growth generated by epigean organisms, while troglobitic species remained unchanged.