Several factors affecting soil structural stability interact in the natural grasslands of the Flooding Pampa (Argentina). This seasonal and flat wetland (usually ponded in winter-spring and dry in summer) has swelling soils, which are affected by seasonal increases in sodium, and continuous grazing by cattle. Our study aimed to determine the period of structural deterioration, the magnitude of deterioration, and the time for recovery in a widely distributed soil of the region. The mean weight diameter of wet-sieved aggregates (MWD) was determined in grazed and ungrazed (exclosure) plots. Aggregate MWD was often lower in the soil under grazing (from 4.4 to 5.1 mm) than in that of the exclosed area (from 4.7 to 5.4 mm). This reduction in aggregate size was attributable to the mechanical shearing action of trampling. Soil water content accounted for 74% of the variation in aggregate MWD under grazing. At low soil water contents, the structure of the grazed soil became less stable. Grazing effects on soil structural stability are significant only in periods when the soil dries. Stocking rates must be regulated at those dry periods.