This thesis investigates the association between breeding system and transposable element (TE) dynamics in 'Daphnia pulex.' TEs are one of the major sources of variation in genomes and 'Pokey,' a class II TE, is a stable element of 'Daphnia' genomes. ' D. pulex' has two reproductive modes, obligate and cyclical parthenogenesis. Using a PCR based approach, TE-Display, 'Pokey' number in 22 cyclic and 22 obligate isolates of 'D. pulex,' and in 46 isolates from eight cyclic populations was determined. I hypothesized that the average number of 'Pokey' fragments per individual would be significantly higher in cyclic than in obligate individuals and this was supported by the results, suggesting that breeding system does affect the dynamics of TEs. This study also shows two new characteristics of 'Pokey': TTAA is not the only site into which 'Pokey' inserts, and ' Pokey' can insert in both the reverse and forward direction in 28S rRNA genes.