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Near real-time data from the MODIS satellite sensor was used to detect and trace a harmful algal bloom (HAB), or red tide, in SW Florida coastal waters from October to December 2004. MODIS fluorescence line height (FLH in W m⁻² μm⁻¹ sr⁻¹) data showed the highest correlation with near-concurrent in situ chlorophyll-a concentration (Chl in mg m⁻³). For Chl ranging between 0.4 to 4 mg m⁻³ the ratio between MODIS FLH and in situ Chl is about 0.1 W m⁻² μm⁻¹ sr⁻¹ per mg m⁻³ chlorophyll (Chl=1.255 (FLH×10)^0.86, r=0.92, n=77). In contrast, the band-ratio chlorophyll product of either MODIS or SeaWiFS in this complex coastal environment provided false information. Errors in the satellite Chl data can be both negative and positive (3–15 times higher than in situ Chl) and these data are often inconsistent either spatially or temporally, due to interferences of other water constituents. The red tide that formed from …