The propagation effect on electromagnetic pulses associated with negative return strokes is evaluated from data observed by direction finders for lightning location equipped with GPS clocks. Nine direction finders constitute a network in the northern part of the Honshu Island of Japan, which locates lightning by a method combining the direction finding and the time-of-arrival (TOA) technique. The delay of the peak of an electromagnetic pulse was evaluated to be negligible in propagation over seawater for distances less than 300 km, and its average was about 1.8 μs in the east-west propagation over land for the distance of about 130 km. The timing error of 1.8 μs may cause maximum location error of about 0.5 km within a TOA lightning-location network, and larger error is anticipated outside of the network. The evaluated propagation delay in this area corresponds to the average ground conductivity of about 0.003 S/m, except for the southwest part with slightly lower conductivity of about 0.002 S/m. These findings were confirmed by an analysis based on ground-truth data obtained by still cameras equipped with precise clocks. The measured delay in the north-south propagation corresponded to the average ground conductivity of about 0.003 S/m. The estimated arrival time of an electromagnetic pulse, based on the arrival time of the peak and the trigger time, gives smaller delay than the simple arrival time of the peak, but the difference is not significant. It would be better to compensate the peak time by evaluating the propagation effect in obtaining an estimate of the arrival time, to improve the accuracy of lightning location by using the TOA technique. ¿ 1998 American Geophysical Union