I investigated the ability of Burkholderia cenocepacia, an opportunistic bacterial pathogen, to adapt to a host. Studies have identified trade-offs associated with environmental adaptation, but few have investigated host adaptation. Consequently, I studied effects of adaptation by B. cenocepacia to onions (Allium cepa) on the ability to kill Caenorhabditis elegans. I hypothesized that adaptation to onions would reduce virulence in C. elegans. I evolved twelve populations of bacteria in onion tissue medium for 500 generations. Then, I quantified fitness differences between evolved and ancestral populations by direct competition, having developed molecular marking techniques to discriminate among competitors. Competitions revealed fitness increases in nine populations. Next, I measured virulence against C. elegans of each population and observed a reduced worm killing ability. I also quantified pleiotropic effects of adaptation related to virulence. In conclusion, I supported that adaptation of B. cenocepacia to one host resulted in decreased virulence in another host.