Treatment of small animals with radiation has in general been limited to planar fields shaped with lead blocks, complicating spatial localization of dose and treatment of deep-seated targets. In order to advance laboratory radiotherapy toward what is accomplished in the clinic, we have constructed a variable aperture collimator for use in shaping the beam of microCT scanner. This unit can image small animal subjects at high resolution, and is capable of delivering therapeutic doses in reasonable exposure times. The proposed collimator consists of two stages, each containing six trapezoidal brass blocks that move along a frame in a manner similar to a camera iris producing a hexagonal aperture of variable size. The two stages are offset by 30° and adjusted for the divergence of the x-ray beam so as to produce a dodecagonal profile at isocenter. Slotted rotating driving plates are used to apply force to pins in the collimator blocks and effect collimator motion. This device has been investigated through both simulation and measurement. The collimator aperture size varied from as the driving plate angle increased from 0 to 41°. The torque required to adjust the collimator varied from , increasing with increasing driving plate angle. The transmission profiles produced by the scanner at isocenter exhibited a penumbra of approximately 10% of the collimator aperture width. Misalignment between the collimator assembly and the x-ray source could be identified on the transmission images and corrected by adjustment of the collimator location. This variable aperture collimator technology is therefore a feasible and flexible solution for adjustable shaping of radiation beams for use in small animal radiotherapy as well as other applications in which beam shaping is desired.