Feather pecking is a behavioural disorder of laying hens and has serious animal welfare and economic implications. One of the several aetiological hypotheses proposes that the disorder results from redirected exploratory behaviour. Variation in the gene encoding the dopamine D4 receptor (DRD4) has been shown to be associated with exploratory behaviour in several species, including in a passerine bird species. We therefore considered DRD4 as a candidate gene for feather pecking. We have annotated DRD4 in the chicken genome and have re-sequenced it in 140 animals belonging to: experimental layer lines divergently selected for high and low propensity to feather pecking; the unselected founder population; and two commercial lines with low and high propensity to feather pecking. We have identified two sub-haplotypes of DRD4 that are highly significantly associated with feather pecking behaviour in the experimental (P = 7.30 x 10(-7)) as well as in the commercial lines (P = 2.78 x 10(-6)). Linkage disequilibrium (LD) extends into a neighbouring gene encoding deformed epidermal autoregulatory factor 1 (DEAF1). The product of DEAF1 regulates the transcription of the gene encoding the serotonin (5-hydroxytryptamine) 1A receptor. Thus, DEAF1 represents another candidate gene for feather pecking. Re-sequencing of five animals homozygous for the 'low-pecking' sub-haplotype and of six animals homozygous for the 'high-pecking' sub-haplotype delineated an LD block of 14 833 bases spanning the two genes. None of the variants in the LD block is obviously functional. However, the haplotype information will be useful to select against the propensity to feather pecking in chicken and to elucidate the functional implications of the variants.