Various beam selectors for selectively placing one of at least two beams of light along the same output path are disclosed. In one aspect, a beam selector receives at least two substantially parallel beams of light. The beam selector includes a plate with an aperture so that when one of the at least two beams is selected for transmission, the beam selector directs only the selected beam along an output path through the aperture. The plate can also serve to block transmission of unselected beams. The output path through the aperture is the same for each of the at least two beams when each beam is selected. Beam selectors can be incorporated into fluorescence microscopy instruments to selectively place particular excitation beams along the same path through the microscope objective lens and into a specimen to excite fluorescence of fluorescent probes attached to a particular component of the specimen.