We report the characterisation of Nocardioides sp. SP12, an atrazine-degrading bacteria isolated from atrazine-treated bulk- and maize rhizosphere soil. Based on 16S rDNA alignment, strain SP12 showed close phylogenic relationships with Nocardioides sp. C157 and Nocardioides simplex. Internal transcribed spacer (ITS) sequences of strain SP12 were longer than those of other Nocardioides sp. and present Ala- and Ile-tRNA unlike Actinomycetales. Nocardioides sp. SP12 presents a novel atrazine catabolic pathway combining trzN with atzB and atzC. Atrazine biodegradation ends in a metabolite that co-eluted in HPLC with cyanuric acid. This metabolite shows an absorption spectrum identical to that of cyanuric acid with a maximal absorption at 214.6 nm. The mass of the atrazine metabolite is in concordance with that of cyanuric acid according to mass spectrometry analysis. Quantitative PCR revealed that the ITS sequence of Nocardioides sp. SP12 was at a lower number than the one of trzN in atrazine-treated soil samples. It suggests that trzN could also be present in other atrazine degrading bacteria. The numbers of trzN and ITS sequences of Nocardioides sp. SP12 were higher in the maize rhizosphere than in bulk soil.http://www.blackwell-synergy.com/doi/abs/10.1016/S0378-1097(03)00168-X