The design and operation of a valved solid‐source of arsenic for molecular beam epitaxy is described. The addition of a throttle valve to a solid‐charge arsenic source allows the arsenic flux to be varied rapidly and controllably during epitaxial growth with a time constant of 2, and provisions for adding an exit aperture ionization gauge for feedback flux control. Low‐temperature photoluminescence from undoped GaAs grown with this source exhibited a neutral donor‐bound exciton peak with full width at half‐maximum of 0.128 meV, and lightly silicon‐doped GaAs exhibited a mobility of 73 000 cm2/V s at 77 K, corresponding to ND =1.3×1015/cm3 and NA =2.0×1014/cm3.