Azaspiracids (AZAs) are a group of lipophilic polyether toxins implicated in incidents of shellfish poisoning in humans, particularly in northern Europe. We isolated an AZA-producing dinoflagellate, later described as a new genus and species Azadinium spinosum, from eastern Scottish coastal waters during a North Sea oceanographic cruise in 2007. In 2008 detailed analysis of plankton from Niskin bottle samples collected on a cruise along the Danish west coast typically showed highest amounts of AZA in the 3-20 µm fraction. We established a number of crude cultures from this size fraction at the station with highest AZA abundance. An AZA-producing clone corresponding taxonomically to the description of A. spinosum was isolated by microcapillary and brought into pure culture. This dinoflagellate clone produces AZA-1, AZA-2 and an isomer of AZA-2 and thus shows qualitatively the same AZA profile as the A. spinosum strain previously isolated from the Scottish east coast. However, the Danish clone, in contrast to the Scottish isolate, produces only a small amount of the structurally as yet undescribed isomer of AZA-2. This confirmation of A. spinosum as a causative dinoflagellate for AZA toxicity essentially explains the lack of correlation of AZA with the abundance and distribution of the previously postulated culprit, the heterotrophic dinoflagellate Protoperidinium crassipes. Evidence of AZA toxins in the plankton from the Danish coast and confirmed association with A. spinosum also further supports the previous findings of the wide biogeographical distribution of these toxins throughout the coastal North Sea, and hence potential risk to shellfish resources and human health.