Kalilo is a linear plasmid associated with senescence in Neurospora. The terminal Eco R1 restriction fragments of this element are linked to a protein component which remains bound despite denaturation with high concentrations of SDS. Following digestion with proteinase K, the 5′ termini of the plasmid remain resistant to lambda exonuclease whereas the 3′ termini are sensitive to exonuclease III, suggesting that the terminal protein is covalently linked. From an analysis of iodinated proteins released by nuclease digestion, the size of the terminal protein was estimated to be 120 kDa. The covalent linkage between DNA and protein was shown to be alkali-labile suggesting that it is a phosphodiester bond. Electron micrographs of the intact plasmid demonstrate that the associated proteins are terminal, and may be involved in replication.