In the spring of 1998, numerous fires from seasonal biomass burning in Central America, mainly in Mexico and Guatemala, produced aerosol particles that were advected into the central plains of the United States. The effects of the fires continued from approximately April 9 through June 11. The most intense smoke concentration was on May 8 as seen from a NASA Shuttle mission. Characteristics of cloud-to-ground (CG) lightning flashes recorded by the National Lightning Detection Network were examined for May 1998 by subtracting the lightning characteristics for May 1995--1997, and 1999. This produces difference value maps with approximately 100 km resolution for polarity, peak currents, and multiplicity. Several significant differences are revealed. The percentage of positive flashes increased by a factor of two. Median first stroke peak currents have been calculated for both positive and negative flashes. For negative flashes, the median peak current decreased during the fire period. For positive flashes, the median peak current increased by over 20 kA in Texas, Oklahoma, Kansas, Arkansas and Nebraska. Mean multiplicity values, or the number of strokes per flash, were calculated. Positive flash multiplicity did not change. Mean multiplicity values for negative flashes, however, changed dramatically. For a region including Texas, Oklahoma, Kansas, and Louisiana multiplicity values for negative flashes decreased from 2.8 to 1.0--1.4 strokes per flash. The data analyzed were a corrected set from which we had deleted positive flashes with peak currents less than 10 kA, thus removing most of the intracloud contamination. Further studies may reveal the relation between the effect of pollution and aerosol size on the characteristics of CG flashes. ¿ 2000 American Geophysical Union