We have discovered plasmids in 5 of 18 diatom species surveyed. In several species, more than one type of plasmid is present. Several of the plasmids show similarity by hybridization to previously characterized plasmids in Cylindrothecafusiformis (J. D. Jacobs et al., unpublished data). Additionally, there is similarity between the plasmids found in C. fusiformis and chloroplast DNA in three diatom species. These results add to the evidence that the plasmids have features of mobile genetic elements.We have recently discovered two small circular DNA plasmids, pCfl and pCf2, in the marine diatom Cylindrotheca fusiformis (9). These plasmids are 4.27 kbp (pCfl) and 4.08 kbp (pCf2) in size. Sequence analysis (8) shows that the plasmids share a large region of significant similarity. They hybridize to each other under low-stringency conditions but not under high-stringency conditions. Under high-stringency conditions, both plasmids hybridize to high-molecularweight (genomic) chloroplast DNA, and pCf2 also hybridizes to nuclear DNA. When coupled with the sequence information, this suggests that substantial portions of the plasmids are present in genomic DNA. The evidence to date is consistent with the hypothesis that the plasmids are mobile genetic elements (8). These intriguing results prompted us to see whether plasmids commonly occur in other diatom species and to determine whether they share similar features.