Blooms of the phytoplankton coccolithophorid Emiliania huxleyi can cause significant changes to both the inherent and the apparent optical properties within an oceanic column. Measurements made within such a bloom off the southwestern coast of England during July 1999 are reported. The multiple scattering properties of the bloom prevented accurate retrieval of absorption (a) and attenuation (c) coefficients with a WETLabs ac-9. Upwelling radiance measurements were similarly affected by the bloom, which caused the sensors to saturate. An optical model has been developed that gives close agreement with the in situ optics when it is used as input to the Hydrolight radiative-transfer model.