Maize variety Zacatecas 58 (Z0), was used to select, after 12 cycles of mass selection for grain yield, an improved population (Z12). Some agronomical traits were characterized at the end of the selected period in both populations. Ribulose-1,5-bisphosphate carboxylase–oxygenase (RuBPcase) activity, phosphoenolpyruvate carboxylase (PEPcase) activity, protein content, and chlorophyll content were measured in the leaves above the ear, throughout the grain filling period (from anthesis to 40 days afterwards). Various agronomical traits associated with increase in yield, such as length and diameter of ear, rows per ear, and kernels per row, were greater in the Z12 than in the Z0 population. Likewise, the Z12 population showed 50% more yield than Z0, although days to anthesis did not increase significantly. During the grain filling period the leaves of the Z12 population had more activity of RuBPcase but not PEPcase compared with the Z0 population. It is suggested that the higher level of RuBPcase activity observed in the Z12 population might be related to its improved grain yield.