The decline of the Tiwanaku state saw the emergence of two new cultures—Pica‐Tarapacá and Atacama—during the Late Intermediate Period in northern Chile. Archeological evidence suggests that both groups practised maize agriculture and pastoralism, but that their interaction zones differed significantly. Marine resources are common at Pica‐Tarapacá sites, even those far from coast, while Atacama sites in the desert oases and precordilleran area seem to have directed their networks towards the highlands. Here we apply stable carbon, nitrogen, and oxygen isotope analysis on human bone and enamel to test dietary patterns and residential mobility at two sites, Pica 8 and Quitor 6, representing the Pica‐Tarapacá and Atacama cultures, respectively. Our results show that diet at the two sites indeed differed: significant but variable consumption of marine resources and maize is indicated at Pica 8, despite being an inland site, while diet at Quitor 6 was based mainly on terrestrial resources. The use of seabird guano and llama dung as fertilizers and extreme aridity may have contributed to the high nitrogen isotope values observed in Pica 8 humans. The δ¹⁸O values in Pica 8 individuals are generally lower than for Quitor in spite of its greater distance from the Andes. All three isotopes suggest the presence of at least five nonlocals in the 30 measured at Pica 8. This evidence for human mobility is consistent with the high levels of trade and interaction observed in the archeological record, and begins to quantify the degree of movement of specific individuals. Am J Phys Anthropol 156:374–387, 2015. © 2014 Wiley Periodicals, Inc.