This is an open-label extension of Study SERES-012. Subjects who had a CDI recurrence in Study SERES-012 within 8 weeks of receipt of study drug, have responded to a course of standard of care (SOC) antibiotic treatment, and who have completed their SERES-012 Week 8 visit will receive an oral dose of SER-109. A treatment regimen of SER-109 is defined as an oral dose of SER-109 in 4 capsules once daily for 3 consecutive days.