The maize (Zea mays L.) glyceraldehyde-3-phosphate dehydrogenase gene 4 ( GapC4) promoter confers anaerobic gene expression in tobacco (Nicotiana tabacum L.), potato (Solanum tuberosum L.) and Arabidopsis thaliana (L.) Heynh. Here we have investigated its expression in hybrid poplar (Populus tremula x P. alba). Our results show that the promoter is not expressed in leaves and stems under normoxic conditions while anaerobiosis induces reporter gene expression in leaves up to a level observed for the STLS-1 promoter from potato that is shown to confer leaf-specific gene expression in transgenic poplar. Anaerobic induction is cell autonomous and requires a CO2 atmosphere and light. As in tobacco, the GapC4 promoter in poplar is wound inducible. The induction by CO2 and light may reflect a natural situation because flooding, a natural cause of anaerobiosis, is often accompanied by high CO2 concentrations in the floodwater. Our results show that the GapC4 promoter is suitable as an anaerobic reporter and as an inducible gene expression system in poplar.