In rainbow trout (Salmo gairdneri Richardson) which had been a c c l i m a t i s e d to the exp e r i m e n t a l salinity and temperature, food intake was maximal in the intermediate salinities of 15.O and 28.0~, less in fresh water and 7.5~ and minimal, by a s t a t i s t i c a l l y s i g n i f i c a n t margin, in 32.5~. There were m a r k e d d a y -t o -d a y fluctuations in food intake. When salinity was abruptly increased by 7.5 or 13.O~, there were decreases in growth rate w h i c h were related to decreases in food intake. Recovery of food intake and growth rate to p r e -i n c r e a s e levels was complete w i t h i n 14 days. A b s o r p t i o n efficiency, in terms of total dry matter, total energy and total nitrogen, was n e g a t i v e l y related to salinity. Total n i t r o g e n was absorbed c o n s i d e r a b l y more e f f i c i e n t l y than either energy or dry matter. C o n v e r s i o n efficiency (K I a n d K 2) was estimated, also in terms of dry matter, energy and total nitrogen, in trout of the O+ and I+ year groups w e i g h i n g from 50 to 150 g. There was a tendency for conversion efficiency to decline with salinity, especially between 28.0 and 32.5~. Dry m a t t e r and energy conversion were s i g n i f i c a n t l y lower than nitrogen c o n v e r s i o n efficiency.