The germination characteristics of seeds of six varieties of sunflower (Helianthus annuus L.) were investigated at constant temperatures ranging from 1.5 to 49.0 degrees C. There were variations among the varieties in the way they responded to temperature. Relationships between germination rate (reciprocal of time to 50% germination) and temperature revealed that base temperature (T-b) ranged from 1.0 to 5.1 degrees C. The ranges for the optimum and maximum temperatures for germination were 34.0 to 36.7 degrees C and 45.5 to 50.9 degrees C, respectively. The practical significance of the cardinal temperatures and other germination characteristics of the seeds are discussed.