Black grouse in Britain have faced contraction of their range and have declined in numbers during the recent decades. As such, the species is a conservation concern in the UK. In order to aid conservation decisions, the terms Evolutionary Significant Unit (ESU) and Management Unit (MU) have been proposed. An ESU is an independently evolving evolutionary lineage defined by being reciprocally monophyletic for mitochondrial alleles, and which is significantly different from other lineages with regard to nuclear alleles, whereas an MU is operationally defined by only the latter criterion. Using mitochondrial sequences and nuclear microsatellite loci, we failed to find evidence that British black grouse is an ESU. However, British black grouse are sufficiently different from continental black grouse both with respect to mitochondrial and nuclear data to regard them as a separate MU. Furthermore, we present genetic data which suggest that British black grouse presently occur in what are probably three demographically independent units (roughly corresponding to Wales, northern England/southern Scotland and northern Scotland), which are genetically differentiated. The two southern units (Wales and northern England/southern Scotland) have lower genetic diversity and show signs of having lost genetic variability.