The bio-optical signatures of harmful algal blooms can be used to define ocean color satellite algorithms. We characterized the bio-optical properties of nutrient-replete cultures of the red tide dinoflagellate Gymnodinium breve and the diatom Thalassiosira weissflogii. We cultured large volumes (900L) of these organisms and performed measurements outdoors over a broad range of chlorophyll concentrations (0.1 to 30µg/L) characteristic of bloom progression. Measurements were made in unialgal tanks and in a mixed culture tank in which the proportion of the two species were varied but total chlorophyll remained constant at 15µg/L. The suite of measurements included HPLC photopigments, particle size distribution, in vivo spectral absorption and attenuation using WETLabs ac-9 and ac-100, filter pad/CDOM absorption, remote sensing reflectance, and backscattering. Preliminary results indicate that spectral absorption of the two species showed a high degree of similarity. Total scattering and backscattering spectra differed substantially between species in unialgal cultures and this difference was evident in the mixed culture tank. An experiment of this nature and scale has never before been successfully completed to our knowledge, and the results provide a valuable and unique data set with potential for ocean color applications.