Chicken eggs are often incubated commercially in complete darkness, although in nature they would receive some light exposure. Light, like other environmental stimuli, can affect the development and behavior of chickens post-hatch. To determine the effect of exposing embryos to light on post-hatch behavioral rhythms, fear and physical asymmetry (an indicator of stress susceptibility), we incubated broiler chicken eggs (N = 664) under either 24L:0D (L), 0L:24D (D), or 12L:12D (LD); the light level was 550 lux. Post-hatch they were raised under a 12L:12D regimen. Physical asymmetry was assessed using a composite asymmetry score determined by middle toe length and metatarsal length and width. Data were analyzed using ANOVA with an LSD post-hoc test. The D chickens were more fearful than LD, as indicated by their responses to induction of tonic immobility (TI) and inversion after catching (INV). At 5 wk of age L (232 ± 24 sec) and LD (213 ± 24 sec) had shorter (P < 0.05) latencies to right during TI than D (306 ± 24 sec); D also wing-flapped more intensely (P < 0.05; 5.7 ± 0.1 flaps/sec) during INV than LD (5.0 ± 0.1 flaps/sec) or L (5.4 ± 0.2 flaps/sec). The D chickens had a higher level of composite physical asymmetry (1.87 ± 0.11mm) than LD (1.57 ± 0.09mm, P = 0.05); they also tended to have more composite asymmetry than L (1.62 ± 0.11mm, P = 0.10). There were no differences in activity rhythms but there was a difference in feed consumption rate for the first 2 h after light onset, with L and LD eating more during this time period. The results of this study indicate that light stimulation during embryogenesis results in long-term effects on fear responses and stress susceptibility as indicated by physical asymmetry.