'Fast hisslers', or very brief bursts of auroral hiss dispersed in the whistler mode, have been found to occur in substorms at Byrd station, Antarctica. Broad band VLF data recorded on tape during breakup phases of substorms were monitored aurally. In two of 19 substorm breakup phases observed at Byrd station, fast hisslers were heard. The data were subsequently spectrum analyzed to permit measurement of whistler mode dispersion, from which the altitudes of their origin were estimated. Some of the fast hisslers exhibited a 'noise' or nonextremal frequency of earliest arrival. This feature permitted an estimate of altitude of origin that is nearly independent of the field line electron density distribution model. Altitudes of origin of 1800-30,000 km are deduced. Some fast hisslers have been observed at phases other than the breakup phase. Fast Fourier transform spectra show that fast hisslers are indistingusihable from dispersed short pulses of band-limited white noise. Incoherent Cerenkov radiation from precipitating auroral electrons is of insuffficent intensity to account for this phenomenon.