Nitrogen (N2) fixation is a globally important process often mediated by diazotrophic cyanobacteria in the open ocean. In 2010, seawater was collected near Cape Verde to identify and measure N2 and carbon (C) fixation by unicellular diazotrophic cyanobacteria. The nifH gene abundance (10(4)-10(6)nifHL(-1)) and nifH gene transcript abundance (10(2)-10(4)cDNAnifHL(-1)) for two unicellular groups, UCYN-A and UCYN-B, were detected. UCYN-A was also identified and quantified (10(4)-10(5)cellsL(-1)) by new probes (UCYN-A732 and UCYN-A159) using Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization (CARD-FISH) assays. The UCYN-A were observed as free cells or attached to a larger unidentified eukaryotic cell. A Halogen In Situ Hybridization-Secondary Ion Mass Spectrometry (HISH-SIMS) assay using the UCYN-A732 probe was applied on samples previously incubated with (13)C-bicarbonate and (15)N2. Free UCYN-A cells were enriched in both (13)C and (15)N and estimated C and N2 fixation rates for UCYN-A were lower compared to co-occurring unicellular cyanobacteria cells similar in size (3.1-5.6μm) and pigmentation to diazotroph Crocosphaera watsonii. Here, we identify and quantify two common co-occurring unicellular groups and measure their cellular activities by nanoSIMS.