Anaerobic and aerobic bacteria were quantitated in respiratory samples across three cystic fibrosis (CF) centres using extended culture methods. Subjects, ages 1-69 years, who were clinically stable provided sputum (n=200) or bronchoalveolar lavage (n=55). Eighteen anaerobic and 39 aerobic genera were cultured from 59% and 95% of samples, respectively; 16/57 genera had a ≥5% prevalence across centres. Analyses of microbial communities using co-occurrence networks in sputum samples showed groupings of oral, including anaerobic, bacteria whereas typical CF pathogens formed distinct entities. Pseudomonas was associated with worse nutrition and F508del genotype, whereas anaerobe prevalence was positively associated with pancreatic sufficiency, better nutrition and better lung function. A higher ratio of total anaerobe/total aerobe colony forming units was associated with pancreatic sufficiency and better nutrition. Subjects grouped by factor analysis who had relative dominance of anaerobes over aerobes had milder disease compared to a Pseudomonas-dominated group with similar proportions of subjects being homozygous for F508del. In summary, anaerobic bacteria occurred at an early age. In sputum producing subjects anaerobic bacteria were associated with milder disease suggesting that targeted eradication of anaerobes may not be warranted in sputum producing CF subjects.