Survey on the geographical distribution and nest density of leaf-cutting ants was conducted in gramineous fields at a total of 64 localities in southern Brazil and Paraguay. Four species of Acromyrmex and four species of Atta were collected: Ac.balzani, Ac. fracticornis, Ac. heyeri, Ac. striatus, At. capiguara, At. laevigata, At. sexdens, and At. vollenweideri. Ac. balzani, At. capiguara, and At. laevigata, adapted to tropical-climatic condition, were densely distributed in the north of the surveyed area, seldom in the centre, and not in the south. Ac. fracticornis, Ac. heyeri, Ac. striatus, and At.vollenweideri, adapted to subtropical-climatic condition, were collected in the southern part of the surveyed area, while only Ac. fracticornis was found also in the northern area. Ants of both types occurred in the centre. A cosmopolitan species, At. sexdens, was collected from almost all the regions surveyed but its density was usually low. The surveyed area was divided into eleven regions, for which the mean nest density of each ant species was calculated. A cluster analysis was applied for these regions, and three groups were classified. One group covered the most northern regions and another covered the most southern regions. The last group covered the central regions. The result of the analysis agreed well with the geographical location of the regions. Comparison of our data with those in previous reports suggests that leaf-cutting ants are expanding their distribution, and that tropical species and subtropical ones may occur in the central regions. The findings imply that any regions in the surveyed area are likely to be invaded by leaf-cutting ants of either the tropical or subtropical species, or both, and may encounter problems with the ants in future ..