The taxonomy of Carex section Ammoglochin is complex due to the faint morphological species boundaries and overlapping ecological niches. This study focusses on species boundaries within the C. arenaria complex, in particular C. arenaria, C. brizoides and C. pseudobrizoides. Carex pseudobrizoides is morphologically very similar to C. arenaria, but also shares some features with C. brizoides, which has sometimes led to the assumption of a hybridogenic origin. We studied the morphology, ecology and distribution of these species in Flanders, and combined this with DNA sequence data (plastid encoded matK and nuclear rDNA ITS) from a large number of specimens. Our results do not provide evidence for a hybridogenic origin of C. pseudobrizoides (although it cannot be rejected either), but instead indicate possible conspecificity or a very recent divergence of C. pseudobrizoides and C. arenaria, which remains undetected by the two genetic markers that were used. Although there are still outstanding questions, our results further improve our understanding of species boundaries in this species complex in Europe, and highlight the need for further investigations using more variable molecular markers.