New data on the breeding of Caspian Tern Sterna caspia in Diawling National Park, Mauritania. A colony of Caspian Terns Sterna caspia was discovered in Aftout Es Saheli, Mauritania, at the periphery of Diawling National Park (16°4l'N 16°22'W), on 20 February 2006. The site was surrounded by water of a mean depth of 20-50 cm. Nests were lined with dead Tamarix twigs, some were concealed below dead tree trunks and others were in old Lesser Flamingo Phoeniconaias minor nests. Breeding was not synchronised: we observed eggs being incubated, eggs hatching, nests with chicks, and large chicks accompanying adults. The number of breeding pairs was estimated at 900. Nests contained a total of c.192 chicks and more than 50 large chicks were seen following adults. Previous studies estimated the number of pairs in Aftout at only 100 and no significant breeding had been recorded in the area since the 1970s. The West African population of Caspian Terns has dramatically increased in recent years and is now estimated at 45,000-60,000 individuals, whereas c.10 years ago there were just c.5,000 pairs.