In the period 1998–2010 the endemic subspecies of the Eurasian Spoonbill Platalea leucorodia balsaci breeding in Mauritania has decreased in numbers considerably. The causes for this decline are unknown. This study aimed to investigate the diet of the species. We analysed faecal material collected in the breeding colonies in 8 different years. The results show that Mauritanian Spoonbills almost exclusively eat shrimp (59.7%) and small fish (35.4%), the latter being dominated by Gobiidae (20.8%), Soleidae (4.8%) and Mugilidae (2.8%). Another 10 fish families were represented in small proportions. Shrimp were quantified on the basis of (parts of) mandibles present in the samples. All prey items eaten by the Spoonbills were extremely small. Diet composition of adult birds and chicks appeared to be similar. There was great variation in diet composition of adults between years, but there was no trend in any of the major diet components over the study period. This indicates that the decline of the Spoonbill population is not correlated with changes in food composition. Our diet study has been of a qualitative nature. Considering the dramatic population decline we plea for a more detailed ecological study of the species, including a quantitative approach of food intake and foraging conditions.