We used in situ radiance/irradiance profiles to retrieve profiles of the spectral backscattering coefficient for all particles in an E. huxleyi coccolithophore bloom off the coast of Plymouth, UK. At high detached coccolith concentrations the spectra of backscattering all showed a minimum near $\sim 550$ to $600\mathrm{nm}$ . Using flow cytometry estimates of the detached coccolith concentration, and assuming all of the backscattering (over and above the backscattering by the water itself) was due to detached coccoliths, we determined the upper limit of the backscattering cross section ( ${\sigma }_b$ ) of individual coccoliths to be $0.123\pm 0.039{\mathrm{\mu m}}^2/\text{coccolith}$ at $500\mathrm{nm}$ . Physical models of detached coccoliths were then developed and the discrete dipole approximation was used to compute their average backscattering cross section in random orientation. The result was $0.092{\mathrm{\mu m}}^2$ at $500\mathrm{nm}$ , with the computed ${\sigma }_b$ displaying a spectral shape similar to the measurements, but with less apparent increase in backscattering toward the red. When ${\sigma }_b$ is computed on a per mole of calcite, rather than a per coccolith basis, it agreed reasonably well with that determined for acid-labile backscattering at $632\mathrm{nm}$ averaged over several species of cultured calcifying algae. Intact coccolithophore cells were taken into account by arguing that coccoliths attached to coccolithophore cells (forming a “coccosphere”) backscatter in a manner similar to free coccoliths in random orientation. Estimating the number of coccoliths per coccosphere and using the observed number of coccolithophore cells resulted is an apparent backscattering cross section at $500\mathrm{nm}$ of $0.114\pm 0.013{\mathrm{\mu m}}^2/\text{coccolith}$ , in satisfactory agreement with the measured backscattering.