During early postnatal ontogenesis, juvenile rainbow trout undergo a number of morphological and physiological changes, passing through various stages of development of body systems and behavioural traits. The most important factor influencing these processes is the fish diet. The present study provides data on external indicators, morphometric and ethological characteristics of juvenile rainbow trout fed with factory starter and frozen feeds. Methods of continuous logging and recording of fish behavioural responses and morphometric methods to analyse juvenile growth were used. It was found that the compound feed had a better effect on the development of the juvenile rainbow trout studied than the frozen feed, despite the fact that the fish started eating the frozen feed earlier.