To investigate the primary structural determinants affecting heat resistance of Clostridium sporogenes spores, electron micrographs of heat-sensitive (D121 degrees C = 0.56 min) and heat-resistant (D121 degrees C = 0.93 min) spores of C. sporogenes were taken with a transmission electron microscope. The mean thickness (+/- standard deviation [SD]) of coat layers and cortex regions of heat-sensitive spores were 82.9 +/- 14.5 and 86.0 +/- 22.7 nm, while those of heat-resistant spores were 106.9 +/- 45.7 and 111.7 +/- 32.1 nm, respectively. The thickness of coat (P = 0.031) and cortex (P = 0.006) showed statistically significant differences, suggesting that heat-resistant spores have a thicker coat and cortex than do heat-sensitive spores. The mean sizes (+/- SD) of cores were 467.0 +/- 88.7 nm for heat-sensitive spores and 460.2 +/- 98.5 nm for heat-resistant spores, respectively, which showed no statistically significant differences. The ratios (+/- SD) of the core size to the sporoplast size were 0.84 +/- 0.05 for heat-sensitive spores and 0.80 +/- 0.07 for heat-resistant spores, respectively, showing statistically significant differences (P = 0.030), which indicated that the ratio is negatively related to heat resistance. Accordingly, the structural components of heat-sensitive spores were severely damaged by heat treatment, whereas those of heat-resistant spores were unlysed under the same conditions. Based on the structural analyses of spores, it was elucidated that the thickness of coat layer and cortex region are significantly correlated with heat resistance of C. sporogenes spores, and that cortex region plays a major role in protecting the spore from heat damage.