The capacity to resist the bactericidal action of complement (C') is a strong but poorly understood virulence trait in Klebsiella spp. Killing requires activation of one or more C' pathways, assembly of C5b-9 membrane attack complexes (MACs) on the surface of the outer membrane (OM), and penetration of MACs into the target bilayer. We interrogated whole-genome sequences of 164 Klebsiella isolates from three tertiary hospitals in Thailand for genes encoding surface-located macromolecules considered to play a role in determination of C' resistance. Most isolates (154/164) were identified as Klebsiella pneumoniae, and the collection conformed to previously established population structures and antibiotic resistance patterns. The distribution of sequence types (STs) and capsular (K) types were also typical of global populations. The majority (64%) of isolates were resistant to C', and the remainder were either rapidly or slowly killed. All isolates carried genes encoding capsular polysaccharides (K antigens), which have been strongly linked to C' resistance. In contrast to previous reports, there were no differences in the amount of capsule produced by C'-resistant isolates compared to C'-susceptible isolates, nor was there any correlation between serum reactivity and the presence of hypermucoviscous capsules. Similarly, there were no correlations between the presence of genes specifying lipopolysaccharide O-side chains or major OM proteins. Some virulence factors were found more frequently in C'-resistant isolates but were considered to reflect clonal ST expansion. Thus, no single gene accounts for the C' resistance of the isolates sequenced in this study.IMPORTANCE Multidrug-resistant Klebsiella pneumoniae is responsible for an increasing proportion of nosocomial infections, and emerging hypervirulent K. pneumoniae clones now cause severe community-acquired infections in otherwise healthy individuals. These bacteria are adept at circumventing immune defenses, and most survive and grow in serum; their capacity to avoid C'-mediated destruction is correlated with their invasive potential. Killing of Gram-negative bacteria occurs following activation of the C' cascades and stable deposition of C5b-9 MACs onto the OM. For Klebsiella, studies with mutants and conjugants have invoked capsules, lipopolysaccharide O-side chains, and OM proteins as determinants of C' resistance, although the precise roles of the macromolecules are unclear. In this study, we sequenced 164 Klebsiella isolates with different C' susceptibilities to identify genes involved in resistance. We conclude that no single OM constituent can account for resistance, which is likely to depend on biophysical properties of the target bilayer.