The carbonate chimneys of the Lost City Hydrothermal Field on the Mid-Atlantic Ridge are coated in thick microbial biofilms consisting of just a few dominant species. We report a preliminary analysis of a biofilm metagenome that revealed a remarkable abundance and diversity of genes potentially involved in lateral gene transfer (LGT). More than 8% of all metagenomic reads showed significant sequence similarity to transposases; all available metagenomic data sets from other environments contained at least an order of magnitude fewer transposases. Furthermore, the sequence diversity of transposase genes in the biofilm was much greater than that of 16S rRNA genes. The small size and high sequencing coverage of contigs containing transposases indicate that they are located on small but abundant extragenomic molecules. These results suggest that rampant LGT among members of the Lost City biofilm may serve as a generator of phenotypic diversity in a community with very low organismal diversity.