The composition and abundance of mycosporine-like amino acids (MAAs) were investigated in the surface waters along a 13,000-km meridional transect (52° N to 45° S) in the Atlantic Ocean (Atlantic Meridional Transect programme: Cruise AMT 18: 4/10/2008-10/11/2008). MAAs were ubiquitous along the transect, although the composition of the MAAs was variable. Highest concentrations were in the far south (below 40° S; MAA >1 μg L(-1)) and in north subtropical equatorial region (NER: 0-25° N; MAA up to 0.8 μg L(-1)). Highest MAA relative to chlorophyll-a occurred in the NER (MAA/chl-a ratio between 2 and 5). MAA/chl-a significantly correlated with the preceding month's mean daily UV dose and with UV-B/UV-A. In the far south, high MAA concentrations coincided with high phytoplankton biomass, high nutrients and a deep mixed layer associated with the austral spring. Here, the phytoplankton community was dominated by micro- and nano-eukaryotes. At the NER, the high MAA/chl-a coincided with low nutrient concentrations, a shallow mixed layer depth (20-70 m) and to a lesser extent to a shallow nitracline (40-90 m). Here, the phytoplankton consisted primarily of picophytoplankton (0-0.2 μm), dominated by the pico-cyanobacteria Synechococcus sp. and Prochlorococcus sp. and by the nitrogen fixing filamentous cyanobacterium Trichodesmium. The low nitrate concentrations (<0.1 μmol L(-1)) at the NER suggest that nitrogen fixation was required for MAA production. Specific MAAs could not easily be assigned to particular groups of phytoplankton and we could not rule out the possibility that MAAs were associated with symbiotic cyanobacteria contained within heterotrophic dinoflagellates or diatoms.