An apparatus for detecting fluorescently-labeled molecules moving in an electrophoretic separation medium. The apparatus utilizes a scanning detection system in which multiple fluorophores are efficiently and simultaneously detected using dichroic mirrors to allow simultaneous detection of fluorescently-labeled molecules in ultrathin gels labeled with several fluorophores and to permit operation at speeds ten times faster than prior art gel separations. The apparatus also utilizes a detection system where lightweight collection optics are in motion while detection optics are fixed in a remote location, thereby allowing exceptionally high-speed scanning.