  Particles with electric charge q < 10^(-3)e and masses in the range 1--100
MeV/c^2 are not excluded by present experiments. An experiment uniquely suited
to the production and detection of such "millicharged" particles has been
carried out at SLAC. This experiment is sensitive to the infrequent excitation
and ionization of matter expected from the passage of such a particle. Analysis
of the data rules out a region of mass and charge, establishing, for example, a
95%-confidence upper limit on electric charge of 4.1X10^(-5)e for millicharged
particles of mass 1 MeV/c^2 and 5.8X10^(-4)e for mass 100 MeV/c^2.
