The present experiment was designed to evaluate the effect of spawning season rearing temperature and post-ovulatory oocyte ageing on egg developmental success in rainbow trout, Oncorhynchus mykiss, assessed as percent survival at the eyeing, hatch and swim-up stages, and as percent occurrence of malformations and ploidy anomalies at yolk sac resorption. Moreover, female weight, egg wet weight, oocyte ATP concentration and ovarian fluid pH, osmolality and protein concentration were measured to investigate possible links with post-ovulatory oocyte ageing and egg developmental success. Thirty three mature females were held at 12 degreesC and 10 at 17 degreesC. Eggs and ovarian fluid were sampled at the time of detected ovulation and 1, 2 and 3 weeks later. Eggs were inseminated with fresh semen pooled from five males and embryo development was monitored until yolk sac resorption. Generally, survival rate decreased and malformation rate increased with an increase in post-ovulatory oocyte ageing. Egg developmental success was positively correlated to female weight and to ovarian fluid pH and osmolality. Ovarian fluid protein concentration significantly increased during post-ovulatory ageing, whereas oocyte ATP concentration remained constant. A significant increase in morphological anomalies was observed as soon as 7 days post-ovulation (dpo), though this effect was not correlated with decreased survival. Also, an increase in spontaneous triploidy was associated with increased post-ovulatory oocyte ageing and high temperature, sometimes in dramatically high proportions (50%).