Decimal reduction times (D values) at 100° C in sterile acid ripe plantain puree were calculated for 2-, 10- and 16-month old spores of Bacillus coagulans and Bacillus macerans. Results indicated that the D100° C values obtained were, respectively, 8.4, 13.2 and 9.2 mm for B. coagulans and 9.7, 9.7 and 9.7 min for B. macerans. According to these data, spore age has no effect on heat resistance for B. macerans. For B. coagulans, however, spore age appears to have an effect on thermal resistance. When 10-month old B. coagulans spores were used, the D100° C values obtained were significantly higher (P<0.01) than the ones for either 2- or 16-month old spores. This possible fluctuation in heat resistance with age of spore may introduce an element of variability that should not be overlooked when B. coagulans is used to determine the adequacy of a thermal process in acid or acidified foods. Since under certain conditions the spores of B. macerans could be more, or at least equally, heat resistant than those of B. coagulans, the question arises whether B. macerans could or should be used in lieu of B. coagulans in determining the adequacy of thermal processes in acid or acidified foods.