This article discusses the composition of prehispanic diets based on the isotopic composition (δ13C, δ15N) of 10 human samples dated between 3600 and 1300 BP from the Cajón Valley in Northwest Argentina. The sample is the only one available for the time and region considered and covers a 2,300-year range. It shows long-term tendencies in the consumption of food resources during a time when people were experimenting with and eventually domesticating plants and animals in the high-altitude Southern Calchaquíes valleys. Results indicate that animals contributed more to the diet than was expected and plants correspondingly less than expected. This proportion was maintained even during the human occupations associated with the agropastoral villages of Cardonal and Bordo Marcial (ca. 2000 BP) when a greater contribution of plant resources—in particular maize—would be expected. This new evidence demonstrates the diversity of productive strategies used by the societies of Northwest Argentina at the beginning of the Formative period. A variety of resources contributed to daily food intake, minimizing the importance of maize in the diet.