TURBULENCE in the interstellar medium transfers energy from parsec-sized regions to much smaller scales, and may be responsible for supporting clouds against gravitational collapse(1). Fluctuations in the electron density, which trace turbulence, occur on scales ranging from 10(6) to > 10(13) cm-the largest range of spatial scales seen in natural turbulence. Despite almost thirty years of study, however, the causes and effects of interstellar turbulence are still poorly understood. Here we present observations of OH masers in the Galactic star-forming complex W49N, which we use as point sources to investigate scattering along the line of sight. The masers’ images are elliptical, and aligned roughly perpendicular to the Galactic plane. This alignment suggests that the magnetic field of our Galaxy influences interstellar turbulence(2,3) by mediating the transfer of energy from large to small spatial scales.