A study was conducted to determine the length of exposure to heat necessary to kill water-imbibing weed seeds at 55 and 60°C. The examination covered ten upland weed species: Solanum carolinense L., S. americanum Mill., Abutilon theophrasti Medic., Phytolacca americana L., Amaranthus spinosus L., A. patulus Bertoloni, Persicaria lapathifolia (L.) S. F. Gray, Panicum dichotomiflorum Michx, Echinochloa crus-galli (L.) Beauv. var. crus-galli and Digitaria ciliaris (Retz.) Koeler. All seeds of each species were killed by exposure to heat at 55°C for 72hr and at 60°C for 24hr except for those of Abutilon theophrasti (dormant seed rate 80%) which were killed by exposure for 120 and 30hr at the respective temeratures.These weed species fell into four groups, categorized by a combination of short-duration heat-tolerance (SDHT) and long-duration heat-tolerance (LDHT). SDHT was determined by the viability of weed seeds when treated at 60°C for 3hr, with low and high subcategories of tolerance. LDHT was detemined by the duration required to kill all the seeds at 1% level of significance when treated at 60°C, into three subcategories (low, medium and high tolerance). Persicaria lapathifolia, E. crus-galli, and D. ciliaris were low for both SDHT and LDHT (Group I), and Amaranthus spinosus and Panicum dichotomiflorum were low-SDHT and medium-LDHT (Group II). Solanum species, Phytolacca americana, and Amaranthus patulus were high-SDHT and medium-LDHT (Group III), while Abutilon theophrasti was low-SDHT and high-LDHT (Group IV). The viability of seeds at short-duration heat treatment for Group I, II and IV was low, but the durations required to kill all seeds were different among the groups. Group III consisted of species for which the viability of seeds at short-duration heat treatment was high, and the fatal duration was medium.For Abutilon teophrasti which required the longest duration of heat exposure to kill all seeds at each temperature, 95% confidence intervals of LD90 values were calculated using the probit method, and were 42 to 58 and 10 to 17hr at 55 and 60°C, respectively.