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International audience The abundance and distribution of cyanobacte-rial diazotrophs were quantified in two regions (Melanesian archipelago, MA; and subtropical gyre, SG) of the western tropical South Pacific using nifH quantitative polymerase chain reaction (qPCR) assays. UCYN-A1 and A2 host populations were quantified using 18S rRNA qPCR assays including one newly developed assay. All phylotypes were detected in the upper photic zone (0-50 m), with higher abundances in the MA region. Trichodesmium and UCYN-B dominated and ranged from 2.18 × 10 2 to 9.41 × 10 6 and 1.10 × 10 2 to 2.78 × 10 6 nifH copies L −1 , respectively. Het-1 (symbiont of Rhizosolenia diatoms) was the next most abundant (1.40 × 10 1-1.74 × 10 5 nifH copies L −1) and co-occurred with het-2 and het-3. UCYN-A1 and A2 were the least abundant diazotrophs and were below detection (bd) in 63 and 79, respectively, of 120 samples. In addition, in up to 39 % of samples in which UCYN-A1 and A2 were detected, their respective hosts were bd. Pairwise comparisons of the nifH abundances and various environmental parameters supported two groups: a deep-dwelling group (45 m) comprised of UCYN-A1 and A2 and a surface group (0-15 m) comprised of Trichodesmium, het-1 and het-2. Temperature and photosynthetically active radiation were positively correlated with the surface group, while UCYN-A1 and A2 were positively correlated with depth, salinity, and oxygen. Similarly, in a meta-analysis of 11 external datasets, all diazotrophs, except UCYN-A were correlated with temperature. Combined, our results indicate that conditions favoring the UCYN-A symbiosis differ from those of diatom diazotroph associations and free-living cyanobacterial diazotrophs.