An investigation was conducted on an Olivier silt loam (fine‐silty, mixed, thermic aquic, Fragiudalfs) at two locations to determine the influence of 6 seasonal temperatures and 4 N rates on head diameter, head weight, tissue NO3concentration and crop yield of 4 head lettuce (Lactuca sativaL.) cultivars. An analysis of covariance was used to establish prediction equations for each dependent variable. Temperature was shown to have a significant curvilinear influence on all crop parameters. There was a significant cultivar X temperature interaction with respect to all dependent variables except tissue NO3concentration. Calculated optimum mean temperatures for maximum head diameters, head weights, and yields for all cultivars were found to range between 17.4° and 18.2°C.