It has been shown that Clostridium perfringens isolates associated with food poisoning carry a chromosomal cpe gene, whereas nonfood-borne human gastrointestinal disease isolates carry a plasmid cpe gene. In addition, the chromosomal cpe gene isolates exhibit greater heat resistance as compared with the plasmid cpe strains. Therefore, the current study conducted ultrastructural measurements of spores from several plasmid and chromosomal cpe-positive C. perfringens isolates. In support of the dehydration mechanism of spore heat resistance, the C. perfringens spore core average size was found to show a negative correlation with D-values for spores obtained at 100 degrees C. Dipicolinic acid (DPA) concentrations assayed for the spores did not correlate well with C. perfringens spore core averages nor with D(10)-values at 100 degrees C. Spore core thickness might be a distinguishing phenotypic characteristic used to identify heat resistance and survival potential of C. perfringens in improperly cooked foods.