This study includes remote sensing data from the same algae bloom in the northern Baltic Proper (north of Gotland) acquired by four different sensors, including one from a radar. All satellite scenes were acquired on July 15, 1997, over the same area within less than two hours. Using this extensive data set, for the first time, an investigation of the usefulness of the fusion of different remote sensing data (particularly by incorporating radar data) for monitoring of an ongoing algae bloom has been performed. The results of our analysis show that the fusion of optical and microwave remote sensing data (even with different spatial resolutions) can yield more information about the biological and oceanic characteristics of sea surface areas.