Populations of T. infestans and T. sordida were studied between September 1988 and April 1989 with a view to discovering if any kind of interaction took place between them while they lives together in the same experimental unit and exploited the same food resource (chicken). The initial age structure for each species was: 27 N1, 7 N2, 11 N3, 3 N4, 8 N5, 4 males and 10 females. The population dynamics nutritional status, predation and gregarious behavior were estimated by means of a monthly census. The colonizing success of T. infestans was greater than that of T. sordida in view of the values obtained: fecundity (146 eggs/female), longevity (157.8 days) and mortality (39.4) compared with the values recorded for T. sordida: 118 eggs/female, 81.1 days and 54.0% respectively. The population growth of T. insfestans followed an exponential model, with a high nymphal recruitment rate, while the T. sordida population was early extinguished. In general, the average weight of T. infestans remained close to the initial values while that of T. sordida declined. Gregarious groups were formed principally in the inferior sector of wall 1 (near the host) with a higher aggregation in T. infestans. These results make it possible to propose the hypothesis of the competitive superiority of T. infestans.