The grain-filling period between pollination and physiological maturity in grain corn is very important for crop productivity. During the early stages of this grain-filling period, the grain has a high water content which later is replaced by organic matter (biomass) as the grain matures and dries. This grain-filling process depends on several factors including genotype and environmental conditions such as temperature and humidity, but there remains much to be understood about this process, especially as affected by genotype. We studied water and biomass changes in the developing grain under field conditions throughout the grain-filling period. The genotypes tested included four land races from central and southern Tamaulipas, Mexico and two commercially distributed genotypes of grain corn, also in Tamaulipas. The experiments were carried out at the UAM Agronomia y Ciencias de la Universidad Autonoma de Tamaulipas during the Fall-Winter of 2004. We found no statistical difference between genotypes for rate of grain-filling during the linear phase and little biomass accumulation during the first 12 days post-pollination. However, after the initial 12 days post-pollination we observed a high linear rate of biomass accumulation, with the duration of this period being statistically different between genotypes. This was followed by a significant reduction in rate of biomass accumulation until physiological maturity was reached. We observed no differences between genotypes in the rate of water loss. The highest content of water was reached at 26-28 days post-pollination with a positive correlation between the amount of water in the grain and rate of biomass accumulation, hence with individual grain weight.