Results are presented from testbeam measurements made with a scintillating fiber preshower detector. The detector was designed to study the early development of electromagnetic showers and the conversion of synchrotron radiation photons. It was constructed with a 0.5 mm transverse granularity, and eight 0.25 radiation length (X0) longitudinal samplings. Data were taken with 50 GeV pions and electrons with energies between 10 and 200 GeV. Measurements, for electrons, of both the energy deposited and the shower width as a function of depth are presented in this paper. In addition, data were taken with a 1.8 m long magnet with a 2.5 T field placed directly upstream of the detector and the synchrotron radiation photons, generated by the electrons traversing it, were observed in our detector. Electron identification efficiencies and pion rejection factors based on the synchrotron radiation signal and the energy deposited in the last sampling layer (1.75 X0) are presented.