Although plasmid-like mitochondrial DNA molecules have been investigated in a number of cultivated plant species, knowledge about their occurrence and behavior in natural plant populations is scarce. In the bladder campion, Silene vulgaris, a common weed of northern Europe, mt-plasmids of three different sizes were detected in a survey of S. vulgaris populations in southern Sweden. Two of the three plasmids usually occurred together within individual plants and showed large variation in frequency between populations. From F_ST-estimates of plasmids, mitochondrial markers, and nuclear markers it was concluded that the plasmids are predominantly maternally inherited in their natural habitat, as observed in greenhouse experiments. The association between mt-plasmids and mitochondrial haplotype was strong, but not complete, in the natural material. These results indicate that the mt-plasmids of S. vulgaris have evolved toward almost strict maternal inheritance.