Against the background of rapidly increasing fish demand and stagnant or declining marine harvest, aquaculture or fish farming has assumed a major role in France which is set to expand. Trout farming in particular has already displayed considerable growth and France is the leading producer in the EEC. Biotechnology holds the key to future changes in trout culture. One such technology, which trials show has potential to reduce production costs, is recombinant trout growth hormone (rtGH). This paper sets out to perform a preliminary ex ante assessment of the possible social benefits from the adoption of rtGH by French trout producers. Several scenarios, based on possible hypotheses of supply and demand growth, are considered. Scenarios assuming an association of the technology with diversification towards more highly processed trout products display the highest estimated welfare gains. A key factor which will determine the outcome of using rtGH is its acceptance by the public. There has already been strong adverse reaction in Europe to the use of genetically engineered growth hormones in meat production and to the use of bovine somatotropin to enhance milk yields. The possibility that there might be a similar response in the case of trout is examined by considering the possibility of a sharp drop in demand. Part 1 of the paper sets out the economic and technological background to biotechnological development of trout farming in France. Part 2 undertakes an ex ante assessment of potential changes in producer and consumer surplus from the adoption of rtGH applying alternative supply and demand shifts in the framework of partial equilibrium analysis.