In 1974 R. de Naurois and F. Roux proposed that the distinct morphology of Eurasian Spoonbills Platalea leucorodia breeding on offshore islands in the Banc d'Arguin, Mauritania, in comparison with the sympatrically wintering northwest European breeding Spoonbills Platalea leucorodia leucorodia, justifies recognition as a separate subspecies Platalea leucorodia balsaci. This proposal is examined here on the basis of variation in nuclear DNA, microsatellites identified earlier for P. minor and P ajaja. We show that there is significant variation between Spoonbills breeding in Mauritania (n = 25) and the sympatrically wintering conspecifics breeding in the Dutch Wadden Sea (n = 105). Of the total genetic variation among the 130 individuals, 6.3% is attributable to variation between the two breeding areas (93.7% of the variation is within breeding areas). Pairwise F_ST values showed low genetic differentiation (FST'S < 0.012) among breeding colonies within The Netherlands. The level of genetic differentiation indicates that the level of gene flow between The Netherlands and Mauritania is much lower (∼4–5 individuals/generation) than among the Dutch colonies on separate Wadden Sea islands. Field observations on individually colour-marked birds from The Netherlands indeed suggest extensive dispersal within northwest Europe, with some introgression of leucorodia genes into the balsaci population. The level of microsatellite distinctiveness between sympatric populations of the two subspecies is similar to that recorded for subspecies of other migrant birds, and as such justifies the subspecies status of the Mauritanian Spoonbill.