Enhancing soil organic C (SOC) is a key factor in semiarid Mediterranean agroecosystems where weak soil structure and low crop yields are common attributes. C sequestration in no-tillage (NT) soils as a result of greater microaggregate formation within macroaggregates and reduced macroaggregate turnover compared to conventional tillage (CT) has been demonstrated in different environments. This study was aimed to show whether NT compared to CT promoted microaggregate formation and microaggregate-C stabilization within macroaggregates in Mediterranean semiarid areas of Northern Spain. The study was conducted on three long-term tillage experimental sites differing in annual rainfall and evapotranspiration, SOC content and silt plus clay content. Although small or no differences in the proportion of macroaggregate-occluded microaggregates (mM) were observed between NT and CT in all three sites, C content associated with microaggregates (mM-C) was greater under NT relative to CT. This result was explained by higher concentrations of intra-microaggregate particulate organic matter in NT compared to CT.