Time variation of the moments of the distribution of reciprocals of time intervals separating successive zero crossings of the pressure-time speech waveform appearing in a moving 10-msec window are highly correlated with linguistic variations in the signal. Digital-computer analysis of such moments for isolated words uttered in a natural environment shows that fricatives, particularly the /s/ phoneme, are discriminated by the sign of the higher odd moments, whereas nasals and liquids are distinguished by the characteristically constant small values of the even moments. Boundaries for certain phoneme classes can be consistently defined and determined to within 20 msec.