Se comparan dos poblaciones burgalesas medievales con cementerios excavados en roca: Palacios de la Sierra (siglos IX-XIII), núcleo con economía basada principalmente en ganadería y explotación maderera en un entorno frío y montañoso, y Santa María de Tejuela (siglos VIII-XI), con economía fundamentalmente agrícola en una planicie y clima más cálido. La hipótesis del estudio es que deberían existir diferencias en su dieta. Se han analizado los isótopos estables de C y N de 101 individuos y la patología máxilo-dentaria de 79. Palacios de la Sierra, muestra una dieta con un elevado consumo de proteínas animales, superior al de Santa María de Tejuela, y menor patología dental en general. Las diferencias son estadísticamente significativas entre ambos sexos en Palacios, donde destaca un elevado consumo de proteínas animales y una menor tasa de patología dental entre las mujeres. Los resultados obtenidos están en consonancia con la hipótesis del estudio. ABSTRACT: This study compares two medieval villages in the province of Burgos (Northern Spain). The first one, Palacios de la Sierra, is a settlement with an economy based mainly on animal husbandry and logging, dating between the 9th and the 13th centuries CE. It is located in a mountainous area characterized by a cold climate. The second site, Santa María de Tejuela, dated between the 8th and the early 11th centuries CE, is located in a flat area, with a temperate climate and its economy is based mainly on agriculture and livestock breeding. The working hypothesis of the study is that there must have been differences in the diet of the two populations given their different way of life and environment. Stable isotopes of carbon and nitrogen as well as maxillo-dental pathology have been analyzed in a total of 101 adult individuals, in which sex and age have been estimated. In total, the sample of Palacios de la Sierra consists of 16 males and 13 females and that of Santa María de Tejuela consists of 44 males and 28 females. Palacios de la Sierra shows a diet based on C3 plants with high consumption of animal proteins, whereas in the diet of Santa María de Tejuela C4 plants were introduced and the protein values are considerably lower. As for dental pathology, differences have been found between the two sites, with the population of Palacios de la Sierra exhibiting lower rates. Regarding the possible differences by sex, these are marked in Palacios, where females have a diet with a higher protein intake than males, and a lower rate of dental pathology. In Santa María de Tejuela, on the other hand, there are no significant differences by sex in the isotopic values, and males have better dental health. The results obtained in Palacios, especially among females, are consistent with those of a diet based on a significant consumption of animal proteins, possibly dairy products, and even similar to those of medieval elites of Castile. As for the results of Santa María de Tejuela, they are similar to those of other medieval rural settlements in the region and clearly different from those of Palacios de la Sierra.