Atmospheric carbon dioxide concentrations (CO₂) are projected to be between 540 and 970 mmol mol^-1, causing an increase in global air temperatures in the range of 1.4 and 5.8°C by end of this century. Rice (Oryza sativa L.) is one of the important cereal crop grown across the world. Most of the rice is currently grown in regions where current temperatures are already close to optimum for rice production, therefore, any further increases in mean temperatures or period of short episodes of high temperatures during sensitive stages may reduce yields. Thus, identifying and developing high temperature tolerant cultivars will be an important task for rice breeders to meet the demand for food in future climates. The objectives of this research were to understand the impact of high temperature stress on rice and to screen rice cultivars for high temperature tolerance. Forty three rice cultivars from different regions of the world were grown at ambient conditions of Gainesville, Florida, and at high temperature (ambient + 5°C) conditions in four different temperature gradient greenhouses at ambient CO₂.  Data on spikelet fertility (percentage of filled grains) at grain filling stage and data on grain yield and total above biomass were measured at harvest. Harvest index was calculated as the ratio of grain yield to total biomass yield. High temperatures significantly decreased spikelet fertility and harvest indices across all cultivars, but the effects varied among cultivars. The decrease in spikelet fertility ranged from about 10 to 60%. Lower spikelet fertility resulted in fewer filled grains per panicle, lower grain yield and decreased harvest index. The cultivars were classified into three groups (susceptible, moderately susceptible and tolerant) based on their sensitivity to high temperature. These results show the presence genetic variability among rice cultivars and scope for genetic improvement towards high temperature tolerance in rice.