A well-defined attenuation band modulated by the rotation of Jupiter has been found in the spectrum of Jovian hectometric radiation using data from the Galileo plasma wave instrument. The center frequency of this band usually occurs in the frequency range from about 1 to 3 MHz and the bandwidth is about 10 to 20 percent. The center frequency varies systematically with the rotation of Jupiter and has two peaks per rotation, the first at a system III longitude of about 50¿, and the second at about 185¿. It is now believed that the attenuation occurs as the ray path from a high-latitude cyclotron maser source passes approximately parallel to the magnetic field near the northern or southern edges of the Io L-shell. The peak at 50¿ system III longitude is attributed to radiation from a southern hemisphere source and the peak at 185¿ is from a northern hemisphere source. The attenuation is thought to be caused by coherent scattering or shallow-angle reflection from field-aligned density irregularities near the Io L-shell. The narrow bandwidth indicates that the density irregularities are confined to a very narrow range of L values (ΔL=0.2 to 0.4) near the Io L-shell. ¿ 1998 American Geophysical Union