Abstract.Improved production of 1+Atlantic salmon,Salmo salarL., smolts can be achieved by taking advantage of the species’ plasticity in response to environmental variables and the timing of physiological events leading to smolting. Smolting commences during the season of declining photoperiod, dependent on reaching a threshold size, after which underyearling populations develop bimodality in length frequency distribution, the upper and lower modal groups (UM, LM) completing smolting at ages 1+and 2+, respectively. We discuss the application of photoperiod and thermal manipulation in relation to the environmental and biological requirements for effective smolting. Such manipulation may be used to promote growth, increasing the incidence of fish attaining threshold smolt size and development of bimodality, followed by completion of smolting the following spring. Growth dynamics and the ‘decision’ to commence smolting are discussed in relation to size thresholds leading to a brief growth surge resulting in UM status. An enzymatic bioassay is described for studying growth during thse short periods of rapid growth associated with the commencement of smolting. Sexual maturation in male parr is discussed in relation to smolting; evidence is presented that parr which become sexually mature during autumn are capable of completing smolting the following spring, together with previously immature fish, provided they reach certain size thresholds near the time of mat