A. The stations 1-5 situated in the head of the Dourduff-estuary (Brittany, France), have been sampled monthly at springtide during the period of May-October 1971. The data obtained this way show a shifting in the populations of Echinogammarus berilloni, Gammarus chevreuxi, and Gammausr zaddachi from the upstream part of the estuary to more downstream and vice versa during the months in which the present investigations have been executed (fig. 3). It appeared that E. berilloni can exist in the head of the estuary, as long as G. chevreuxi and G. zaddachi are few in number or absent (i.e., May-September). When in September the latter two species repopulate the most upstream part of the estuary, E. berilloni disappears from this river section and was found only farther upstream. Because it has been demonstrated that E. berilloni can sustain salinities up to 80% seawater (Vincent, 1966), the increased salinity in the head of the estuary, during the equinoctial springtides in autumn, can not explain the disappearance of this species in that part of the river. The reproduction period of E. berilloni falls during May and June. B. On several places in the estuary of the Dourduff samples have been taken both in the main stream and in the brooklets discharging into it. This sampling resulted in a survey of the distribution of gammarids in the estuary in correlation with some environmental factors (figs. 6 and 7). C. A microgeographical analysis has been executed in the non-estuarine part of the Dourduff and its ramifications. The distribution of species in this part of the river system is shown in figure 5.