1. The migration of two sympatric estuarine species has been studied over a period of one year in the estuary of the river Dourduff, a small river draining into the Bay of Morlaix (Brittany, France). One species, Gammarus zaddachi, is a boreal species and the other, Gammarus chevreuxi, is a lusitanian species. Both species live near the limits of their distribution area. 2. Both species show a migration pattern with an anadromous and a catadromous phase. The migration is an active process. The animals use the tidal movements for their upstream and downstream migration. 3. Migration activities of both species show a diurnal rhythm, the activity peak occurring at night. 4. Downriver migration of both species can be observed throughout the whole year. Upstream migration, however, only takes place during periods with high water spring tide. During these periods the stream reverses direction and water with higher salinities enters the limnic parts of the river. 5. Per night the maximum numbers of anadromous and catadromous migrating individuals caught in a net of 30 X 50 cm were 30,000 and 800,000 respectively for G. chevreuxi. For G. zaddachi these numbers are considerably lower. 6. In both species the composition of the samples from upstream and downstream catches were different. In general, upstream migrating animals are smaller and younger than downstream migrating individuals. 7. G. zaddachi invades the limnic reaches of the river only when water temperatures drop below 6—8° C, a situation occurring for a period of only a few weeks. 8. G. zaddachi reproduces only during the winter months when water temperatures are relatively low. Reproduction takes place in areas with constant or at least periodical influence of mixohaline waters. In the limnic reaches of the Dourduff reproduction is never observed. 9. G. chevreuxi migrates farther upstream than G. zaddachi. It can stay in completely fresh water for more than four months. 10. G. chevreuxi can reproduce during the whole year. Reproduction has never been observed in completely fresh areas, although downstream migrating ovigerous females can be found. 11. In a series of laboratory experiments, survival rates and reproduction capacity were determined under various different temperature/salinity conditions. 12. Temperature has a strong influence on the salinity tolerance of G. zaddachi. At 5° C the mortality rate in river water is comparable to that under optimal salinity conditions. At higher temperatures the mortality sharply increases at lower salinities. 13. Increasing salinity as well as decreasing temperatures have a positive influence upon the reproduction rate of G. zaddachi. In rearing experiments no reproduction has been observed in limnic (= natural) water of the Dourduff, at 5° C. 14. For G. chevreuxi the situation is different. At 5° C survival is low at all salinities. Under brackish conditions (0.8‰ and 8.0‰ Cl) a rise in temperature results in a higher survival rate. In fresh water however (0.08‰ Cl), rise of temperature causes a higher mortality rate. 15. In rearing experiments G. chevreuxi does not reproduce in completely fresh water. Under mixohaline conditions reproduction has been observed at all temperatures examined. 16. The reproduction cycle of the two species is shown in diagrammatic models.