Long-day photoperiods are considered as an effective managerial tool in manipulating somatic growth and reproduction in a number of fish species. Effects of three different artificial long-day photoperiods on somatic growth and gonadal development of Nile tilapia (Oreochromis niloticus L.) were investigated in this study. Swim-up fry with a mean initial weight of 0.06 g were exposed to 24L:0D, 20L:4D and 18L:6D artificial photoperiods and ambient light regime (control) for 24 weeks at 27.0 +/- 1 degrees C. The effect of photoperiodic manipulation was only detectable and statistically meaningful during fingerling stage. Long-day photoperiods resulted in significantly higher mean final weights and specific growth rates (SGR) than natural light regime. The highest mean final weight (24.94 +/- 0.45 g) and SGR (3.46 +/- 0.03% day(-1)) were obtained under 24L: OD photoperiod. Mean female gonadosomatic index (GSI) and mean oocyte size were significantly lower in fish maintained under continuous light regime (24L:0D) than those of 20L:4D, 18L:6D treatments and control. The highest gonadosomatic indices were recorded in control female and males. Mean oocyte diameter in fish exposed to continuous light was measured as 1.05 +/- 0.06 mm with the bulk of the oocytes (60.0%) in pre-vitellogenic stage (<= 1.20 mm). On the contrary, oocyte size and size distribution of oocytes in 20L:4D, 18L:6D photoperiod groups and control were indicating a more advanced oocyte development stage, i.e. vitellogenic (1.2-1.8 mm) and post-vitellogenic stages (> 2.1-2.4 mm). Basically, results obtained support the idea that continuous artificial lighting may be influential on enhancing somatic growth and delaying gonadal development in Nile tilapia during fingerling stage. (c) 2006 Elsevier B.V. All rights reserved.