The Brazilian Northeast is dominated by the Caatinga, a xeric scrub forest characterized by "semiarid" climatic conditions, with annual precipitation often below 600 mm, and an unpredictable rainy season. North of 13 degrees S, the Atlantic Forest forms a narrow coastal strip, of only a few dozen kilometers in width, with a relatively humid climate. Both biomes have been subjected to intense anthropogenic habitat fragmentation over the past few decades, although this process has different implications for each ecoregion. In Sergipe, the geographic ranges of two titi species, Callicebus barbarabrownae and Callicebus coimbrai, appear to correlate with the Caatinga and Atlantic Forest, respectively. Titis are relatively rare in the Caatinga, in terms of both the number of populations and their density. In the lower Rio Sao Francisco basin, 41.2 % of Atlantic Forest fragments (n = 17) had titi populations, whereas only 28.1 % of Caatinga sites (n = 32) had titis. In the ecotone between the two (the Agreste), however, no populations were found (n = 21), suggesting a lacuna in their distribution. Population surveys at sites representative of the distribution of the two titi species indicate that the density of C. coimbrai may be two to four times higher than that of C. barbarabrownae. Overall, the data appear to indicate that while the survival of titi populations in the Caatinga may be dependent on factors such as fragment size and the presence of groundwater, the survival of the populations of the Atlantic Forest may be more closely related to more random factors such as the history of colonization.