Abstract 1. 1. The fluorescence of aqueous suspensions of Chlorella, Scenedesmus, Euglena and spinach chloroplasts is preferentially polarized in a plane perpendicular to an external magnetic field of 10 kG or more. The ratio of the fluorescence intensity viewed perpendicular to the field to the intensity viewed parallel to the field varies from 1.03 to 1.57. 2. 2. The suspensions also exhibit dichroism and anisotropic wavelength-dependent light scattering effects which are induced by the magnetic field. The dichroic maximum may nearly coincide with the absorption maximum of the bulk pigments and in some cases is shifted to the red by selective light scattering. 3. 3. It is concluded that the dichroism and fluorescence polarization are due to a preferred orientation of the chlorophyll porphyrin rings, and the plane of the lamellae, perpendicular to the field. 4. 4. If it is assumed that the magnetic field does not reorient individual chlorophyll molecules, then these results imply that chlorophyll in vivo possesses a higher degree of orientation than previously thought. 5. 5. It is shown for Chlorella that the magnetic field induces a reorientation of the entire cell. 6. 6. The physical basis of these effects can be adequately explained in terms of an anisotropy in the diamagnetic susceptibility of the cell components. 7. 7. Magnetic field induced orientation can be used to study the optical properties of a large number of suspended oriented cells in vivo.