The 16S rDNAs from 38 strains of the genus Nocardioides, two Aeromicrobium species and Terrabacter tumescens were directly sequenced and then analysed, The mean nucleotide similarity value between the type strains of validly described Nocardioides species was 96.1+/-3.0%. The mean nucleotide similarity value between the type strains of validly described Nocardioides species and the two Aeromicrobium species was 93.7+/-1.4%. T. tumescens was distantly related to the genera Nocardioides and Aeromicrobium. The mean intraspecific nucleotide similarity value of 16S rDNA sequences from Nocardioides albus was 99.5+/-0.5%. The mean intraspecific nucleotide similarity of 16S rDNA sequences from Nocardioides simplex was 100%, except for N. simplex strains ATCC 13260, ATCC 19565 and ATCC 19566, which were shown not to be members of the genus Nocardioides. 'Nocardioides flavus' strains IFO 14396(T) and IFO 14397, and 'Nocardioides fulvus' JCM 3335(T) showed a 16S rDNA similarity value of 100% with Nocardioides luteus KCTC 9575(T) and Nocardioides albus JCM 5854. 'N. fulvus' IFO 14399 shared its highest 16S rDNA similarity with Nocardioides sp, ATCC 39419 at 99%, 'N. fulvus' IFO 14399 and Nocardioides sp, ATCC 39419 formed a phylogenetic lineage distinct from the genera Nocardioides and Aeromicrobium, 'Nocardioides thermolilacinus' strains IFO 14335(T) and IFO 14336 displayed a close relationship to the genus Streptomyces. From 16S rDNA sequence analyses, it is considered that some strains that have been attributed to the genus Nocardioides should be taxonomically re-evaluated.