The ability of Candida parapsilosis to grow in the presence of high levels of ethidium bromide (EB) has been explored to study the effects of this intercalating dye on DNA in vivo. By employing confocal microscopy we have determined that EB penetrates the cellular membranes and binds rapidly to the nucleolus, whereas mitochondrial DNA becomes stained after a longer exposure to this dye. No detectable staining of the nucleus has been detected under these conditions. Electrophoretic studies of both undigested and restricted DNAs confirm that the nuclear DNA is unaffected by high levels of EB, with the exception of the rDNA-bearing chromosome that undergoes significant structural alterations in the presence of EB. Moreover, the hybridization signal with the rDNA probe is proportionally reduced in samples obtained from cultures grown in the presence of EB, suggesting that the average copy number of rRNA genes in these cultures may be affected. In striking contrast to other fungal species, the linear organelle genome in C. parapsilosis retains its structural and functional integrity in the presence of high concentrations of EB.