Technical Abstract: Accumulated temperature is a good indicator of development in cereals and in some forage grasses. In the transitional zones of the U.S.A. accumulated temperature may be useful in guiding management of pastures during the change from warm to cool seasons, and vice-versa. It is not clear, however, that accumulated temperature-plant development relations are consistent over a wide temperature range. The influence of average temperature and diurnal temperature range on leaf appearance in three cool-season forage grasses was assessed under four temperature regimes in a controlled environment chamber. Leaf appearance on seedlings of Italian ryegrass (Lolium multiflorum Lam.), tall fescue (Festuca arundinacea Schreb.) and tall wheatgrass (Elytrigia elongata (Host) Nevski) was measured for daily average temperatures of 7.5 and 15 C and with a diurnal differential of 5 or 15 C. Under these regimes mainstem cumulative leaf appearance appeared to be a linear function of accumulated temperature and on average over species was not significantly affected by either average temperature or daily temperature range. Preliminary estimates of phyllochron are 60 growing degree days (GDD) for Italian ryegrass, 84 GDD for tall fescue and 89 GDD for tall wheatgrass.