Selected characteristics of cloud-to-ground lightning were analyzed for twenty-five mesoscale convective systems (MCSs) that occurred within the range of the National Severe Storm Laboratory direction-finder network during the warm season of 1986. Positive ground flashes comprised 6% of ground flashes in MCSs versus 4% in warm-season thunderstorms overall. For negative ground flashes the distribution of estimated peak currents from MCSs appeared almost identical to the distribution from all thunderstorms. The distribution of estimated peak currents varied more from MCS to MCS for positive ground flashes than for negative ground flashes, and the distributions for positive ground flashes in MCSs usually appeared much different from those for storms overall. We divided the spectrum of peak current distributions for positive ground flashes of individual MCSs into three categories: (1) the small-amplitude category (mode of 9--13 kA; few flashes with >40 kA), (2) the middle-amplitude category (mode of 13--17 kA; many flashes with >40 kA), and (3) the large-amplitude category (mode of 51--56 kA). MCSs in the large-amplitude category occurred throughout the warm season; those in the small-amplitude category, primarily during April, May, and September; and those in the middle-amplitude category, during June--August. The maximum total ground flash rate per 30-min period in each MCS tended to be smallest in the large-amplitude category. Median peak currents for all 30-min periods of MCSs in our analysis converged on a relatively narrow range of values when flash rates of a given ground flash polarity increased to large values. For negative ground flashes the median peak currents converged on 25--30 kA in all MCSs. For positive ground flashes the median peak currents converged on one of two narrow ranges: roughly 10 kA for the small-amplitude category and 60 kA for the large- and middle-amplitude categories. ¿ 1998 American Geophysical Union