A series of experiments were performed in order to determine the effects of varying water temperature on the maturation of female rainbow trout subjected to photoperiodic manipulation. Long-short photoperiod regimes, i.e. long days of LD18:6 followed by an abrupt change to short days of LD6:18, were used to advance or delay maturation to summer months, producing commercially desirable out-of-season eggs. At the same time, these fish were exposed to two different seasonal water temperatures supplied by river (range 0-20.5 o C) or borehole (range 7.0-10.5 o C) sources.The photoperiod regime was seen to have the primary effect on altering the timing of maturation and appeared similar, irrespective of the prevailing water temperature. However, water temperature had a modulating effect on time of maturation and ovulation; fish showing an ability to delay the timing of final maturation and ovulation, when temperatures were at extremes, high or low. Extreme temperatures, also, had a major effect on the later stages of ovary development and subsequent egg quality. When the major time of spawning was advanced to July-August, when river water temperatures regularly approach 20 o C, a dysfunction in ovarian development occurred, and no viable eggs were obtainable in these conditions. When the maximum water temperature was reduced to 16 o C, fish stripped in a normal fashion, but egg quality was significantly reduced. Possible alterations in farming practice in order to improve egg survival in such situations are discussed.