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Studies of carbon storage in rangelands are important because of their ample world distribution and their potentially high contribution to worldwide carbon sequestration. The objective of this study was to determine carbon stocks on semiarid grasslands of northern Mexico under grazing and grazing-exclusion for six years. The areas of study were 1) El Salado, located on the north part of the state of San Luis Potosi, and 2) La Soledad, located in the south of the state of Nuevo Leon. The concentration of C in vegetation and soils was determined. Vegetation was sampled every three months for two years to determine aerial biomass production and variation. At the end of the experiment, representative plants were collected to determine the proportion of aerial biomass and root biomass.  Likewise, soil samples from different depths were collected to determine organic C in the laboratory. Biomass production was generally related to the variation in precipitation in El Salado, but not in La Soledad. A high C accrual from aerial biomass was obtained from areas excluded to grazing (323 kg/ha) compared to areas under grazing (141 kg/ha), but only in one of the areas or study (El Salado). On the other study area (La Soledad), grazing did not produce significant differences in C accrual. The lack of response to grazing and precipitation changes in La Soledad are perhaps related to intrinsic soil characteristics. Thus, we conclude that differences in grazing pressure are important in determinations of C sequestration but that soil characteristics can have also an important effect. Our study areas can accumulate high amounts of soil C (between 30,000 kg/ha and 50,000 kg/ha). These amounts are higher than some others reported in arid locations. The factors responsible for this high C accumulation should be further studied.