A Podospora anserina longevity mutant was identified with a temperature-sensitive phenotype for senescence. This mutant, termed TS1, grew for over 3 m at 27°C, but when shifted to 34°C, it underwent senescence between 10 and 18 cm. A previously described senescence-associated plasmid, a senDNA, derived from the mitochondrial genome, was not detected in TS1 at 27°C but was present in senescent cultures at 34°C. A similar result was observed in progeny strains obtained by crossing the TS1 mutant with a wild-type strain. Other mitochondrial excision-amplification DNAs in addition to a senDNA were also observed in the senescent cultures. Most were derived from a specific region of the mitochondrial genome. These results provide evidence that asenDNA is involved in TS1 senescence and suggest that this plasmid may play a role in the formation of other mitochondrial excision-amplification plasmids.