The recent explosion of newly reported eukaryotic transposable elements is illustrating their widespread nature, their ultimate importance as agents of evolution, and the genome as a dynamic entity. My thesis characterizes a transposable element, 'Pokey', found in the ribosomal DNA of 'Daphnia'. Through sequencing of full-length 'Pokey ' elements from 'Daphnia pulicaria', I have shown that it is related to 'piggyBac' and thus, is a member of the TTAA-specific family of DNA transposons. A phylogenetic analysis of the 3' -end of 'Pokey' elements from the subgenus ' Daphnia' showed that 'Pokey' persists via strict vertical transmission and has existed for long periods of evolutionary time, which is unusual for DNA transposons. 'Pokey' inserts into a region of the large subunit ribosomal RNA gene occupied by other elements and it seems likely that this region is an ideal genomic location for the evolution of insertion sequences.