: and often infect the airways in cystic fibrosis (CF). Because registry studies show higher prevalence of vs. in older patients with CF, a common assumption is that replaces over time. In vitro, can outgrow and kill . However, it is unknown how rapidly replaces in patients with CF. : We studied a longitudinal cohort of children and adults with CF who had quantitative sputum cultures. We determined the abundance of and in CFU/mL. We determined the duration and persistence of infections and measured longitudinal changes in culture positivity and abundance for each organism. : Between 2004 and 2017, 134 patients had ≥ 10 quantitative cultures, with median observation time of 10.15 years. 124 patients had at least one positive culture for , and 123 had at least one positive culture for . Both species had median abundance of > 10 CFU/mL. Culture abundance was stable over time for both organisms. There was an increase in the prevalence of / coinfection, but no decrease in prevalence within individuals over time. : and are abundant in CF sputum cultures. Contrary to common assumption, we found no pattern of replacement of by . Many patients with CF have durable long-term coinfection with these organisms. New strategies are needed to prevent and treat these infections.