The purpose of this dissertation is to evaluate the proposition that plasmids can be maintained in bacteria populations as parasites, i.e., by infectious transfer while conferring a selective disadvantage to their hosts (relative to plasmid-free cells). Towards this end, I have extended existing theoretical and experimental studies of the population dynamics of plasmids to consider (1) surface growth, (2) the effects of environmental factors (antibiotics) on rates of plasmid transfer, and (3) conjugative R-plasmids of eight different incompatibility groups. I have developed (1) an experimental method for the quantitative study of population growth and plasmid transfer rates in surface cultures of bacteria, the Surface Slide System, (2) a protocol to estimate the plasmid transfer rate parameter in liquid batch culture, the "end-point method," and (3) a mathematical model of bacterial growth and plasmid transfer in surface populations. The highest rate of plasmid transfer in surface culture obtained when the bacterial population was a confluent layer of micro-colonies. Under these conditions, the kinetics of plasmid transfer fit a simple mass-action model developed for bacteria in liquid culture. When the surface population forms fewer dispersed colonies, less transconjugants are produced per donor and recipient cell. The cost (measured as the decrease in the population growth rate) on the E. coli K12 host strain of carrying each plasmid, was similar in liquid and surface seasonal habitats and substantially less than previously reported for IncFII plasmids in chemostats. The surface model provided a reasonably good fit with the data observed in SSS culture, despite the fact that this model is only a simple first approximation of the complex process of growth and plasmid transfer on surfaces. This study provided evidence in support of the longstanding hypothesis that plasmid fertility is augmented by the presence of antibiotics. In the absence of antibiotic-mediated selection, the two antibiotics tested, ampicillin and kanamycin, augmented the transfer rate parameter of IncFII plasmids by as much as ten-fold. Several of the plasmids studied in serial transfer experiments could become established as parasites in populations with stationary phase densities on the order of 10$\sp8$ cells ml$\sp{-1}$. For most plasmids, the invasion conditions were broader in surface than in liquid culture. Despite this, I argue that in natural populations, a parasitic lifestyle is unlikely.