Planktothrix rubescens is sadly famous for producing microcystins (MCs), which are powerful hepatotoxins. During the winter 2005/06, P. rubescens has been found in the Pozzillo, Nicoletti, Prizzi and Garcia reservoirs, Sicily; in 2008 it was also detected in SS. Trinità di Delia and Castello reservoirs. Indeed, during periods of low shortwave irradiance such as winter, when light weakly penetrates water column and the water cools, P. rubescens filaments float up to the surface, forming red-colored blooms. Ancillary meteorological measurements highlighted low air temperatures between two frosts in December 2007 and February 2008, with a simultaneous reduction in the incoming total solar radiation, which probably triggered P. rubescens floating up to water surface. The manuscript reports empirical relationships to calculate the cell density of P. rubescens. A basic cell density relationship was set up using in situ data and satellite images (MODIS Terra) collected simultaneously. In particular, water samples were collected for cells counting and MCs detection, whereas Red and Near-Infrared bands of MODIS at 250 m provided a good dataset for modeling an empirical relationship, given also their high temporal resolution. Spectroradiometric campaigns were also carried out in February/March 2008 to characterize the spectral reflectance of water with and without P. rubescens bloom. A further cross calibration, using MODIS as reference data, allowed to setup the empirical relationships to be applied to other sensors such as Landsat ETM+, MERIS Envisat and Sentinel-2. Among those, Landsat ETM+ have an appropriate spatial resolution for monitoring Sicilian reservoirs (characterized by an average extension of few kilometers) although images are currently acquired in SLC-off mode, thus compromising the suitability of such data for an operational monitoring. MERIS had a sufficient spatial resolution (300 m) but it was lost with Envisat (on May the 9th, 2012). Sentinel-2, once operational, will provide a suitable dataset. Generally, the retrieved empirical equations tend to overestimate the actual values for low (or absent) surface filaments.