Here, we describe the development of an oomycete-specific primer pair for amplification of the cytochrome b region in plant pathogenic species that span the order Peronosporales (Phytophthora spp., downy mildews). Because of the high number of variable sites at both inter- and intra-specific levels this marker provides a powerful tool for population genetics and phylogenetic studies in this taxa. We also demonstrate its potential compared with other oomycete-specific mitochondrial markers currently available.