The avian W chromosome shares many features with the mammalian Y chromosome: it is small, mostly heterochromatic, and filled with large repetitive arrays. No gene has so far been assigned to the W chromosome in any bird species and, as a practical consequence, a general tag for avian gender identification on the molecular level is lacking. Here I describe the isolation of a chicken homologue to the mouse chromo-helicase-DNA binding (CHD) gene which encodes a protein involved in global regulation of transcriptional activation on the chromatin level. The avian CHD gene exists in two genomic copies, one of which (termed CHD-W) was located on the W chromosome in all non-ratite species investigated. The gene displays extreme levels of sequence conservation since chicken CHD-W and mouse CHD are 82.9% and 95.6% identical at the nucleotide and amino acid level respectively. Molecular sexing can be accomplished in probably all non-ratite birds by hybridizing Southern blots with CHD probes. PCR-based gender identification is also demonstrated. A general system for avian sexing should facilitate many studies of behaviour, evolutionary ecology, genetics, and evolution.