The formerly widespread but now Vulnerable dorcas gazelle Gazella dorcas has declined markedly during the last 2 decades. The semi-captive population in M'Sabih Talaa Reserve, west-central Morocco, is genetically distinct and may be the last remnant of the Moroccan dorcas gazelle G. dorcas massaesyla. The last estimate of the size of this population was c. 200, in 1996. To assess the current population we used line-transect distance sampling to survey this population over five seasons from April 2008 to August 2009. The estimated population declined from 113 to 87 individuals from April to November 2008, increased to 111 in February 2009 (primarily because of recruitment), declined to 99 from February to April 2009, and to 95 by August 2009. These results indicate a decline of nearly 50% in < 15 years. The main conservation actions required for this subspecies are the suppression of poaching and feral dogs, the creation of other reserves to receive dorcas gazelles from M'Sabih Talaa (to avoid loss of the single population from a catastrophic event), and the implementation of a genetic management programme.