Acinetobacter baumannii is an opportunistic pathogen which causes a range of life threatening infections, primarily within the health care setting. A major virulence determinant of A. baumannii is the presence of a polysaccharide capsule forming a barrier around the bacterial cell. To investigate how capsule contributes to the pathogenic success of this bacterium two acapsular mutants were constructed in A. baumannii ATCC 17978. The ΔitrA2 mutant lacks an initial transferase gene involved in the first step of capsule biosynthesis whereas Δwzy lacks the polymerase needed to form a complete polysaccharide chain. These isogenic strains were subjected to a number of in vitro assays to determine the role capsule plays in persistence in the environment and evasion of the host immune response. When grown at pH 7.5, the ΔitrA2 and Δwzy strains lagged slightly compared to WT. At lower pH (pH 5.5), growth was reduced for all strains and the ΔitrA2 strain was severely incapacitated. In contrast, when subjected to osmotic stress (15% sucrose) growth rate of the Δwzy derivative was notably slower than that of WT and ΔitrA2. Both the ΔitrA2 and Δwzy strains displayed significant increases in cell surface hydrophobicity and biofilm formation compared to WT. Lastly, capsule may protect A. baumannii from the host immune system as the acapsular ΔitrA2 and Δwzy strains showed a two- and 1000-fold increase in lysozyme and complement sensitivity compared to the WT strain, respectively. This implies that abrogation of capsule production at different stages of synthesis can differentially affect responses to various stressors; further work is needed to delineate precise mechanisms involved. This study is the first to define a role for capsule in A. baumannii survival in adverse environments and highlights the importance of capsule in evasion of the host immune system.