The size of harvester termite Hodotermes mossambicus (Hagen) (Isoptera: Hodotermitidae) populations on the clay-rich red-soils of semi-arid grasslands in southwestern Zimbabwe in any particular dry season was found to be strongly influenced by the annual rainfall received the preceding rainy season. The size of the termite population of the antecedent dry season also had an important influence. Populations were found to build up over successive dry years and crashed following a season of substantially above average rainfall to fall further after the next rainy season which was also above average. Termite populations and foraging activity also fluctuated considerably within seasons. There was an increase as the dry season progressed but foraging fell dramatically at the end of the season and remained at low levels throughout the summer months. Foraging was confined to degraded patches in the red-soil veld mosaic in the early dry season with expansion into the other grass communities occurring in the late dry season. Fluctuations in harvester termite populations had a marked effect on the extent of the foraging area and hence on the amounts of grass cover remaining in the degraded, ecotone and sound veld communities of the red-soil mosaic at the end of the dry season. Termite foraging activities serve to maintain the grass sward in a vigorous condition by preventing excessive accumulation of dead foliage on ungrazed sites and they had an important influence on the botanical changes which occurred within each grass community of the mosaic. The main effect was to favour those grass species such as Heteropogon contortus and Themeda triandra, that are intolerant of excessive top-hamper (standing dead grass) and litter accumulation.