The rainbow trout (Oncorhynchus mykiss) is a seasonally-breeding teleost which typically spawns for a 6-8 week period each year. The consequent seasonality of egg production restricts the profitability of fish farming; year groups of fish all reach marketable size at the same time and farm facilities are either over or under utilised depending on the time of year. Since these problems can be avoided by the production of out-of-season eggs, techniques designed to modify the time of spawning offer considerable commercial advantage. Of the photoperiod regimes available to modify spawning time those utilising continuous light (LL) are particularly attractive because neither time control or blackout are required. However, year-long exposure to LL can produce erratic spawning. This study examines the ability of short (⩽2 months) periods of LL to advance and delay maturation in the rainbow trout.