E scherichia coli sequence type (ST) 131 is the quintessential example of a successful, global, antimicrobial-resistant, high-risk clone among human bacteria (1,2). Currently, ST131 is the most common global extraintestinal pathogenic E. coli (ExPEC) clone; up to 30% of all ExPEC, 60%-90% of fluoroquinolone-resistant ExPEC, and 40%-80% of ExPEC with extended-spectrum β-lactamases [ESBLs] belong to ST131 (3,4). Population genetics indicate that ST131 consists of different clades (5): clade A contains serotype O16:H5 and fimH41, clade B contains mostly serotype O25b:H4 and fimH22, and clade C contains serotype O25b:H4 and fimH30. Clade C is divided into 2 subclades: C1/H30R (associated with fluoroquinolone resistance) and C2/H30Rx (associated with fluoroquinolone resistance and bla CTX-M-15). A novel ST131 C1 subclade, known as C1-M27 with bla CTX-M-27 , was reported in Japan (6). ST131 is the most dominant and most antimicrobial-resistant among E. coli causing bloodstream infections in Calgary, Alberta, Canada, infecting mostly the elderly in long-term care centers (7). Previous molecular epidemiology studies from the same region showed that ST131 was relatively rare among ESBL-producing and fluoroquinoloneresistant E. coli during the early 2000s but showed a major increase toward the end of the 2000s (8,9). However, limited information is available regarding the changes in population dynamics of ST131 clades over extended periods, especially among nonbiased E. coli isolates in large, well-defined, geographic regions. To address this issue, we conducted a retrospective cohort study that characterized ST131 clades responsible for bloodstream infections in Calgary over an 11-year period (2006-2016). Investigating trends of ST131 clades over long periods by using a populationbased surveillance approach will aid in clarifying the evolution of this clone and help with designing superior prevention strategies (3,10).