The duration of grain fill in wheat (Triticum aestivum L.) is one of the determinants of kernel weight. The purpose of this research was to investigate the genetic diversity for duration of grain fill in winter wheat. In one experiment the genetic diversity for duration of grain fill and the influence of temperature on heading, anthesis and duration were assessed in 175 cultivars and lines of diverse plant type and origin planted at the Purdue Agronomy Research Center. The effect of temperature on the rate and duration of grain fill was further assessed in two cultivars, Fundulea and Caldwell, over 4 yr. In a second experiment Fundulea and Caldwell were grown in the field under altered day length (12, normal, 17 h) to manipulate date of anthesis. Duration of grain fill in the 175 entries averaged 28 ± 0.9 d. Heading occurred on 17 different dates, anthesis on 13 different dates and grain maturity on 17 different dates. The average daily temperature (ADT) during grain fill ranged between 21.2 and 22.5 °C. Comparisons of the ADT through grain fill for Fundulea and Caldwell over 4 yr indicated a 0.83-d reduction in duration for each degree rise in ADT. Leaf area, plant height, and tiller number were similar for cultivars exposed to 17 h, 12 h, or normal day lengths. Anthesis was 5 d earlier and the ADT during grain fill was less (20–21 °C) in plants exposed for 17 h compared to those exposed for 12 h (22 °C). Weight of 1000 kernels was significantly higher in plants with early anthesis dates, but yield was not significantly changed by the day length treatments. There appeared to be genetic differences in temperature sensitivity during reproductive development which could be exploited in selecting cultivars suitable for early anthesis. The general similarity of duration for all experiments suggests that environmental constraints imposed by latitude and selection for yield, have led to a narrow germplasm base regarding duration of grain fill.Key words: Triticum aestivum, average daily temperature during grain fill, duration of grain fill, anthesis, heading