A molecular-beam source utilizing instant flux adjustment for growth rate control is presented. The design uses a two-filament pyrolytic boron nitride (pBN) heater constructed with eight heated exit aperture holes, masked by an aperture-mating pBN closed end cylinder. The rotatable pBN mask opens and closes the effusion cell apertures to provide mechanical control of the source flux. This adjustment is provided by a rotational manipulator that translates rotary motion through the vacuum environment to the mounting journal of the pBN mask cylinder. RHEED oscillation changes in GaAs homoepitaxial growth shows an effectively instantaneous change of nearly an order of magnitude in the growth rate. (C) 2004 American Vacuum Society.