  Searches for transient astrophysical sources often reveal unexpected classes
of objects that are useful physical laboratories. In a recent survey for
pulsars and fast transients we have uncovered four millisecond-duration radio
transients all more than 40{\deg} from the Galactic plane. The bursts'
properties indicate that they are of celestial rather than terrestrial origin.
Host galaxy and intergalactic medium models suggest that they have cosmological
redshifts of 0.5 to 1, and distances of up to 3 gigaparsecs. No temporally
coincident x- or gamma-ray signature was identified in association with the
bursts. Characterization of the source population and identification of host
galaxies offers an opportunity to determine the baryonic content of the
Universe.
