Karenia mikimotoi is a dinoflagellate common in the Eastern North Atlantic Ocean, particularly in summer. Recent K. mikimotoi blooms in Western Ireland (Silke et al., 2005), Scottish waters (Davidson et al., 2006), and the English Channel (Vanhoutte-Brunier et al., 2008) have been reported and described. This dinoflagellate is feared as it may impact the marine animal population directly, through its haemolitic cytotoxin, or indirectly through hypoxia during the degradation of the bloom when its biomass is high. In 2003, a major K. mikimotoi bloom, visible from space, occurred from the end of June to the beginning of August offshore in the Western English Channel. The bloom started between Northern Brittany and Cornwall. It then developed eastward as the thermal stratification set up and favoured its habitat, progressing easterly with the season. Despite the considerable cell concentration level, the damages were limited as the bloom was mainly offshore. No apparent damage was reported in the Channel Islands and only some dead fish and visible stress in the growth of some shellfish species, for instance Pecten maximus, were reported. Some K. mikimotoi blooms are at too great a depth, or are not large enough to be visible from space. However, some reach such an extent and have a sufficiently high concentration of pigment and particles in the surface waters that they can be observed by space-borne Ocean Colour sensors as SeaWiFS, MODIS/AQUA a,d MERIS. At this size and density of bloom it may also be possible in some conditions to discriminate K.mikimotoi from other species (Miller et al., 2006).