Biological media are not usually thought capable of guiding light because they do not exhibit large nonlinear optical responses, but a new study by Zhigang Chen of San Francisco State University and colleagues changes this picture. The team sent a 50 μm-wide green laser beam into a 4 cm-long tube of seawater, upon which the beam broadened to 650 μm, largely due to normal diffraction. Then a strain of cyanobacteria called Synechococcus, which do not move on their own and thus could not actively enter or leave the beam, were added to the tube. At low power (0.1 W) the beam diffraction doubled, but at 3 W the beam constricted to a 0.2 mm needle – with almost all bacteria left unharmed. The data could only be explained by taking into account radiation pressure, which is more important in cells with internal structures. Similar results with E. coli and human red blood cells suggests analogous self-focusing could provide ways to send light through normally opaque tissues for treatment or imaging.