Diatoms are monocellular micro-algae provided with external valves, the frustules, made of amorphous hydrated silica. Frustules present patterns of regular arrays of holes, the areolae, characterized by sub-micrometric dimensions. In particular, frustules from centric diatoms are characterized by a radial disposition of areolae and exhibit several optical properties, such as photoluminescence variations in presence of organic vapors and photonic-crystal-like behaviour as long as propagation of electromagnetic field is concerned. We have studied the transmission of coherent light, at different wavelengths, through single frustules of Coscinodiscus Walesii diatoms, a centric species characterized by a diameter of about 150 μm. The frustules showed the ability to focalize the light in a spot of a few μm2, the focal length depending on the wavelength of the incident radiation. This focusing effect takes place at the centre of …