Bacteria of the genus Rhodococcus can degrade a wide range of organic pollutants and catalyse many useful biotransformations. There is a need for improved tests to identify Rhodococcus species. PCR-based methods for species identification offer advantages in terms of speed and accuracy over traditional methods and can allow direct detection of microbes in environmental samples., PCR tests, using primers targeted at species-specific sequences in the 16S rRNA gene, were successfully developed for R. globerulus, R. erythropolis, R. opacus and R. ruber. These tests gave positive results with all or most strains of target species but did not generally cross-react with other species. Cases of apparent cross-reaction were shown to be due to prior misclassification of strains of R. opacus as R. erythropolis and of strains of R. ruber as R. rhodochrous. A simple and rapid method for the extraction and purification of DNA from soil was developed and successfully applied to the PCR detection of indigenous R. erythropolis in an environmental sample. Cell lysis in the samples was achieved by lysozyme and sarkosyl treatment, aided by freeze-thaw cycles. Removal of humic compounds inhibitory to PCR was accomplished by CTAB treatment with solvent extraction and, if necessary, passage of extracts through Sepharose CL-6B in a spun-column format. Extracts prepared using a tris-EDTA buffer were much clearer than those prepared using a sodium phosphate buffer, indicating lower levels of humic compounds. A detection limit of 104 cfu g-1 of soil was achieved and the use of a secondary PCR allowed detection of 1 cfu g-1.