Linear DNA plasmids are common among filamentous fungi and plants in which they are generally associated with mitochondria. In contrast, the linear DNA plasmids of yeasts appear to be cytoplasmic and to have a specific genetic organization. In these respects the yeast linear plasmids can be regarded as a separate group of plasmids. A survey of plasmids among about 1800 strains covering practically all the currently accepted species of yeast (about 600) has provided a preliminary, overall view of the nature and frequency of yeast plasmids. Linear DNA plasmids were found at a frequency of 1–2%, with more than 20 cases of new linear plasmids discovered in a large variety of species. The same survey showed that circular DNA plasmids are characteristically confined to a few genera and species. The linear DNAs have many features in common with the linear killer DNAs of Kluyveromyces lactis and form a large family of multicopy, small DNAs disseminated among yeast species.