Benthic diazotrophic cyanobacteria are an integral part of typically nutrient limited tropical coral reefs. Despite accumulating reports of high abundances from various areas, there is a lack of knowledge on the role of specific populations for the nitrogen budget of these ecosystems. The diversity, abundance and diazotrophic capacities of benthic cyanobacterial assemblages in the coral reefs of Curacao, Netherlands Antilles were investigated in this study. Identification was achieved by a polyphasic taxonomic approach, relying on morphotypical and genetic characterization, while abundances were measured in a benthic survey of four different sites on the leeward side of the island. Diazotroph diversity was assessed via amplification of nifH gene fragments. Nitrogen fixation and primary production were studied over a diel cycle by parallel application of the acetylene reduction assay and stable isotope tracer methods (15N2 and 13CO2). The most common species were Oscillatoria bonnemaisonii, Oscillatoria sancta, Lyngbya sordida and Hydrocoleum glutinosum. Contrary to previous reports on these species, nitrogen fixation was stimulated by light in all studied populations and coincided with carbon fixation when both were measured. A maximal nitrogenase activity of 31 nmol C2H4 μg Chl a-1 h-1 was recorded for a mixed population of O. Bonnemaisonii and H. glutinosum. NifH gene diversity analysis of this community showed a dominance of cyanobacterial sequences. The acetylene/15N2 reduction ratio was up to 15 fold higher than the theoretical value, suggesting a substantial release of fixed nitrogen. At 3 m depth at the site Pest bay, where cyanobacterial mats reached their maximal benthic substrate coverage of 12 %, nitrogen fixation associated with them was estimated as 1.14 mg N m-2 reef d-1. This study indicates that benthic cyanobacteria have the potential to supply fixed nitrogen in this ecosystem. Furthermore, daytime nitrogen fixation by Oscillatorian cyanobacteria seems to be more widespread than previously thought.