||(Not controlled OCR) Every summer, since 1983, important mortalities of Atlantic salmon smolts occur in net pen farmings in Brittany (France) (Harache and Gaignon, 1986). These mortalities, ranging from 30 to 87 % of stock, are in the way of the economic development of this activity. They take place between mid-June and the end of July. Most of the smolts are sluggish and without appetite. Looking for a viral or bacterial aetiology by the veterinary services was often unfruitful. In order to investigate a possible role of environment, a set of parameters was monitored and compared in two farming sites displaying very different mortality rates.