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High temperature stress during seed filling in controlled environments reduces soybean [Glycine max (L.) Merrill] seed germination and vigor, but the effect of high temperature in the field has not been determined. Seeds of two soybean cultivars (Hutcheson, maturity group [MG] V, and DP4690, MG IV) were produced in the field in Kentucky, Mississippi, Arkansas, and Texas in 2000 to 2002. Air temperature during seed filling was monitored and brown (mature) pods were harvested, hand threshed, and all shriveled and abnormal seeds were removed before determining standard germination and vigor (accelerated-aging germination). Mean maximum temperatures during seed filling (growth stage R5 to R7) ranged from 24.0 (Kentucky) to 37.6°C (Texas). When seed lots infected with Phomopsis longicolla (Hobbs) were removed from the analysis, standard germination and accelerated-aging germination (AA) decreased as mean maximum temperature during seed filling increased, but the decrease was significant (P = 0.05) only for Hutcheson. Standard germination of Hutcheson declined linearly (r2 = 0.49) from near 100% at 24°C to 85% at 36°C, while the decrease in AA was curvilinear (r2 = 0.86) and germination reached 11% at 36°C. Seeds of Hutcheson were more sensitive to high temperature than seeds of DP4690 and seed vigor (AA) was much more sensitive to high-temperature stress than was standard germination. Our findings support the results of experiments in controlled environments by demonstrating that high temperature during seed filling in the field, without seed infection with P. longicolla or physical injury, reduced soybean seed germination and vigor. [ABSTRACT FROM AUTHOR]