Planetesimals are thought to form within the dense dust disks that surround young stars(1), although this process has not been observed directly, In contrast, the presence of dust disks around older, main-sequence stars can be used to infer that planetesimal formation has indeed occurred; dust is rapidly depleted in circumstellar environments, so a continuous supply of dust by the sublimation and collisional erosion of pre-existing planetesimals(2,3) is required to maintain a disk over the lifetime of the central star. The composition of the dust in such disks should provide clues regarding the composition of the planetesimals, and the structure of the disks (for example, gaps in the dust distribution) could be linked to physical interactions with unseen planets, To date, only one main-sequence star-beta Pictoris-has been shown to have a dust disk that can be resolved optically(4). Here we report the optical image of a candidate dust disk surrounding a main-sequence binary stellar system, BD+31 degrees 643, If the existence of this dust disk is confirmed by future observations, it would imply that binary stars may possess stable environments for planetesimal formation.