Based on the analysis of a total of 138,905 T. infestans and 8,921 P. megistus captured in the county of Bambuí, during a period of several years, a study was made of the variations of the percentage of evolutive forms of each species in the different months of the year. In the case of T. infestans the predominance of larvae occurred in November, that of nymphs in February-April, and for the adults in April-July. For P. megistus, the largest occurrence of larvae, nymphs and adults wepe observed, respectively, in March, July and November. While the predominance of adult forms of T. infestans occurs in the dry and cool season, and that of the larvae in the hot and humid season, exactly the contrary is observed in relation to P. megistus. The highest occurrence of acute cases of Chaga's disease occurs in summer. The curves corresponding to the percentages of young forms (larvae and nymphs) and adults of P. megistus show a very constant aspect of its intersection in September, when the adult insects become more numerous than the young. In the case of T. infestans this intersection does not occur, propably on account of the appearance of young forms during the whole year, although there are periods in which they appear with higher frequency. The realization of similar studies in different regions will stablish the season of the year in which the sprays will preferably be made, i. e., when the larvae begin to appear.