Radio pulses of short duration (<0.15 μs) have been observed at a frequency of 44 MHz (λ=6.8 m), in coincidence with extensive cosmic-ray air showers having ∼5·106 particles. The energies of these pulses ranged from ∼10−12 erg (receiver noise level) to ∼10−9 erg. The preliminary results reported briefly elsewhere (19,27) were consistent with the hypothesis that the observed pulses arose as a result of enhanced Cerenkov radiation at low frequencies (10). However, following the treatment of Kahn and Lerche (16), it is evident that the observed energy of the radiation is also consistent with charge separation effects in the earth's magnetic field. Experiments at a higher frequency (150 MHz, λ=2m), and a tentative analysis of the pulse-height spectrum, suggest that the radiation observed is enhanced by effects of mutual coherence among the particles in the shower. The paper concludes with a discussion of other alternative radiation mechanisms.