The aim of this study was to determine the frequency of erm genes coding for macrolide resistance among clinical isolates of non-pigmented rapidly growing mycobacteria (NPRGM) and to evaluate their importance in phenotypic resistance. Broth microdilution susceptibility testing was performed for all NPRGM tested. A PCR assay with consensus primers was used to evaluate the presence of erm genes among the 167 clinical isolates studied, which belonged to nine species of NPRGM; erm genes were detected in all nine species and 109 strains were erm-positive. The highest percentage of erm-positive isolates was found among Mycobacterium mageritense (100%) and the lowest among Mycobacterium mucogenicum (14%). The MICs of macrolides were found to be lower for erm-negative isolates (MIC(90): 2 mg/L) than for erm-positive isolates (MIC(90): 16 mg/L), although in some cases high MICs were found for erm-negative isolates. The finding that erm methylases are present in the majority of the species of NPRGM analysed in this study is not in agreement with conventional susceptibility studies. It therefore appears necessary to use a combination therapy to treat infections caused by NPRGM.