The objective of this study was to measure the in situ spectral reflectance of lake water that contains a bloom of Microcystis, a species of cyanobacteria. Reflectance spectra (350–2,500 nm) of lake water near a boat dock in Upper Klamath Lake, Oregon, were collected with a portable spectrora-diometer on a cloud-free day with sunlight as a source of illumination between 0845 to 0915 hours, Pacific Daylight Time (PDT) on 17 August 2004, at a near-normal angle of observation. The averaged spectrum of the lake water containing the Microcystis bloom exhibits reflectance maxima from 550–590 nm and near 710 nm wavelengths and reflectance minima near 630 nm and 675 nm wavelengths. The reflectance gradually decreases from 810–1,000 nm and has very low reflectance in the 1,000–2,500 nm wavelength region. Our results show that the spectral reflectance of Microcystis at this stage of its bloom remains low for wavelengths longer than 1,000 nm in the near-infrared region of the spectrum. These spectral results have implications in selecting the spectral ratios and refining the algorithms that will be used to estimate phycocyanin content using satellite models. Microcystis is the predominant species of cyanobacteria blooms in Lake Erie, which makes these spectral data as important to the Great Lakes as it is to Upper Klamath Lake. Satellite algorithms have been published that have mapped phycocyanin, a pigment more uniquely associated with cyanobacteria than is chlorophyll a, in Lake Erie with data from LANDSAT TM bands 1, 3, 4, 5, and 7, and the reflectance spectra reported here are the first that cover the entire spectral range of all those LANDSAT TM spectral bands for a Microcystis bloom.