Rates of leaf appearance were evaluated for three maize (Zea mays L.) hybrids grown at two day/night temperatures (20/17 °C and 30/27 °C) in environmentally controlled growth facilities, and for 10 maize hybrids grown in the field. Differences in rate of leaf appearance occurred among genotypes and among phases of development for maize hybrids grown both under controlled-environment and field conditions. Differences in mean rate of leaf appearance between two hybrids common to both experiments were approximately 15% in both the field and the controlled-environment experiments. In addition, hybrid × phase-of-development interactions were highly significant.Key words: Zea mays L., rate of development, temperature, genotype × phase-of-development interaction