Lettuce (Lactuca sativa) is one of the most important U.S. fresh-market vegetables with year-round consumption. For winter markets, lettuce is produced in the southwestern states with plantings in early fall when soil temperatures are high. Seed germination of lettuce, however, is inhibited by soil temperatures over 25 °C. The objective of this study was to test the emergence and growth of five lettuce cultivars using two seedlots produced in winter or summer to provide information for improving stand establishment at high temperatures. Seeds of five cultivars (`Empire', `Parris Island Cos', `Waldmann's Green', `Prizehead', and `Dark Green Boston') produced in Yuma, Ariz., during summer or winter months were used. Seeds were planted in a greenhouse mix in plastic trays and grown in a growth chamber at 23, 25, 30 and 35 °C. After 4 weeks, number of emerged plants, number of leaves, height, fresh weight, dry weight, and leaf area were measured. At lower temperatures (23 and 25 °C), more plants emerged and plants developed more rapidly than at higher temperatures (30 and 35 °C). More plants emerged of `Empire' and `Parris Island Cos' than of `Waldmann's Green' or `Dark Green Boston'. Growth varied greatly for the different cultivars. At 35 °C, only `Empire' winter seed emerged. At other temperatures, summer seed lots generally were better than or equal to winter seedlots. This information suggests that seeds developed during the summer are more vigorous at emergence than those developed during the winter.