A bloom of the non-heterocystous diazotrophic cyanobacterium Trichodesmium erythraeum Ehrenberg is reported in the Canary Islands Archipelago during August of 2004, the warmest period of a meteorological series recorded by the National Institute of Meteorology (Spain) since 1912. Samples showed massive occurrences of T. erythraeum (1000 filaments ml–1) in different sectors of northern and southern waters off the central Canary Islands. Water analyses also showed a relatively low presence of dinoflagellates and diatoms. Quasi-true colour satellite images of dust storms, elevated sea surface temperature (the warmest satellite-derived record), chlorophyll a and geostrophic current fields showed satellite-derived optical positives of Trichodesmium in an African upwelling advective, jet-drifting westward current off the south Canary Islands. Analyses for cyanotoxins using HPLC found microcystins, which was confirmed by immunoassay, at concentrations from 0.1 to 1.0 µg microcystin-LR equivalents (g–1 dry weight of bloom material). A T. erythraeum bloom such as that observed in August 2004 in the NW African Upwelling does not appear to have been recorded for the area previously. The bloom may have developed due to the exceptionally warm weather and/or