Response of corn (Zea mays l.) to day1ength has not been investigated in Puerto Rico. Five genotypes, two temperate (Ohio S9 and Ohio S10), two local (Mayorbe1a and Diente de Caballo), and hybrid X304C (PBH304C) were evaluated every 90 days during a 2-year period at Mayaguez and Isabe1a, Puerto Rico. Planting dates were March 21, June 21, September 21 and December 21. Daylight hours (DL) for the first 60-day growth periods corresponding to the four planting dates were: 746, 771, 687 and 669 hours, respectively. Significant effects of location (L), planting date (PD) and genotype (G) and significant interactions (L x PD and PD x G) were observed for grain yield (GY) and most traits studied. The genotype overall response was higher at Mayaguez compared to the Isabe1a location. Genotypes showed a quadratic response to PD. Genotypes responded to increases in day1ength up to the June planting (771 light hours). The highest GYs, 5971 and 5469 kg/ha, were obtained by PBU304C at Mayaguez and Isabela, respectively. Diente de Calla showed the strongest response to PD and the best yields were obtained by planting before the initiation of long days.