This thesis is a continuation of the study of a linear, mitochondrial plasmid of Claviceps purpurea. The molecular expression and function of this plasmid were studied. It remained difficult to identify the terminal proteins of these linear genetic elements. Experiments to transfer linear plasmids between strains of Claviceps purpurea and Neurospora crassa showed that such transfers do not occur. The author concludes that the study of the linear plasmid of Claviceps purpurea strongly suggests that these genetic elements are most probably of viral origin.