ECOSPOR III is a Phase 3, multicenter, randomized, double-blind, placebo-controlled, parallel-group study of the safety, tolerability, and efficacy of SER-109 versus placebo in adult subjects 18 years of age or older with recurrent CDI, defined as: a history of ≥ 3 CDI episodes within 9 months, inclusive of the current episode. This study is designed to demonstrate the superiority of SER-109 versus placebo to reduce recurrence of Clostridium difficile infection (CDI) in adults who have received antibacterial drug treatment for recurrent CDI (RCDI), based on the proportion of subjects experiencing a CDI recurrence requiring antibiotic treatment up to 8 weeks after initiation of treatment. Approximately 320 subjects with a history of CDI, diarrhea and a positive C. difficile toxin test result on a stool sample, who have responded to standard-of-care (SOC) antibiotic treatment will be enrolled. Subjects will be randomly assigned, in a 1:1 ratio, to 1 of 2 treatment groups (Treatment Group I [SER-109] or Treatment Group II [Placebo]) and stratified by age (<65 years; ≥65 years), as well as antibiotic regimen for the qualifying episode (vancomycin; fidaxomicin).