The migratory activity of Gammarus zaddachi and Gammarus chevreuxi is investigated in a current chamber, in which a tidal cycle can be simulated. In fresh running water the number of animals drifting with the current is of about the same magnitude as the number of animals actively swimming against the current. A diurnal periodicity is recorded in the migratory activity. The migratory activity is influenced by the population density and the food supply. In the current chamber a tidal cycle can be simulated by varying the factors current velocity, current direction, salinity, and temperature. The combination of a decrease in current velocity, followed by a slow current in the opposite direction, an increase in the salinity, and a rise in the temperature of the medium causes a significant increase in the activity of both species investigated. An increase in only one of the environmental factors mentioned above can also produce an increase in the migratory activity, but less pronounced than when all the factors coordinate in simulating a complete tidal cycle. Both G. zaddachi and G. chevreuxi react in a similar way on the simulation of a tidal cycle. In both species no great differences have been found between juveniles and adults with regard to the migratory activity. The results of the simulation experiments are discussed in connection with the migration cycles of G. zaddachi and G. chevreuxi as recorded in the field.