The migration of G. zaddachi in La Slack, a small stream on the French Channel coast, and some physico-chemical properties of the river are discussed. In laboratory experiments, the influence of the Cl- and Ca++ concentration on the survival-rate and reproduction of G. zaddachi was investigated. A raise in the Cl- concentration of the medium caused a significant improvement of the survivalrate. A raise in the Ca++ concentration also gives, although not significant, a higher survival-rate.