The possession of plasmids was for a long time recognized only in the bacteria. It is now evident that plasmids, or replicative forms of DNA structurally and experimentally comparable to bacterial plasmids, exist in eukaryotic organisms as well. Such plasmids are in fact common among fungi and higher plants. The present review is undertaken to provide a comprehensive account of the data available on plasmids found in eukaryotic organisms. This review will not consider plasmids of prokaryotic origin, even though certain bacterial plasmids, such as the tumor-inducing (Ti) plasmids of...