Sets of sole larvae and juveniles were subjected, under DD, LL or DL conditions, to a variety of constant salinities, salinity gradients and cyclic variations of salinity. Animals were observed during several days in cylindrical vertical tanks and their swimming activity was recorded automatically with opto‐electronic IR barriers. Analysis of actograms, periodograms and actotaxigrams demonstrated effects of salinity on behaviour. Salinity variations induced an exogenous rhythm of activity and changes in the distribution patterns of larval and juvenile sole. Nevertheless, sole activity and distribution depended more on nycthemeral light variations than on salinity variations. The ecological consequences of salinity variations in the settlement of juvenile sole in coastal and estuarine areas are discussed.