Plankton samples obtained by the Norwegian Polar Institute (August, 2010) in an area north of Svalbard contained an unusual abundance of tropical and subtropical radiolarian taxa (98 in 145 total observed taxa), not typically found at these high latitudes. A detailed analysis of the composition and abundance of these Radiolaria suggests that a pulse of warm Atlantic water entered the Norwegian Sea and finally entered into the Arctic Ocean, where evidence of both juvenile and adult forms suggests they may have established viable populations. Among radiolarians in general, this may be a good example of ecotypic plasticity. Radiolaria, with their high species number and characteristic morphology, can serve as a useful monitoring tool for pulses of warm water into the Arctic Ocean. Further analyses should be followed up in future years to monitor the fate of these unique plankton assemblages and to determine variation in northward distribution and possible penetration into the polar basin. The fate of this tropical fauna (persistence, disappearance, or genetic intermingling with existing taxa) is presently unknown. The current event may not be unique, nor a consequence of global warming, because analyses of sediment samples suggest that several natural pulses of warm water of this kind occurred in the prior century and, indeed, there may be more in years to come.