The traditional transplanted and flooded rice cropping system is threatened by water shortage and labor cost increase in many Asian countries and is currently evolving toward direct-seeded and water-saving systems. However, yield penalties generally accompany this evolution and ongoing research targets their mitigation. In a previous research, slower plant development rate was observed in aerobic crops, possibly caused by the increased plant density. This possible factor was tested in experiments conducted at IRRI's farm, Los Baños, Philippines, in 2012 and 2013. One elite variety was direct-seeded at three plant distances (6, 10, and 20 cm × 20 cm) and sprinkler-irrigated to keep the soil water potential above -10 kPa. Weekly measurements were made to dissect crop and plant physiological differences. Data from previous experiments on tillering in the greenhouse of four varieties and in fields in aerobic and flooded conditions were added to strengthen the results from the current experiment. The last upper leaves of the main stem appeared faster at higher plant density, thus increase plant density was not the cause for the slower leaf appearance in aerobic than in flooded crops. Plants at higher plant density also had earlier panicle initiation, earlier earing, and 0.8 leaf less than plants at low density. The number of outgrowing tillers was linked to the number of leaves on the main stem by a variety-specific relationship that is independent from plant density and water management. Tiller density was regulated by the cessation of the outgrowth of tillers triggered by the onset of stem elongation at 20-cm plant distance and occurred earlier at shorter distances probably in response to the high root density. Grain yields were significantly higher at higher plant density (6-cm and 10-cm plant distances) and correlated with a higher tiller density. Thus for aerobic crops, seedling density higher than 50 plants m-2 remains the way to get high yields for the type of var