On June 22, 2011, a hospital located near Bordeaux in the Gironde department reported eight cases of bloody diarrhea, including two cases of hemolytic uremic syndrome (HUS) in adults, to the Aquitaine Regional Health Agency. Initial investigations identified a common factor among these cases: attendance at an open house at a recreation center in Bègles on June 8, where a cold buffet featuring sprouted seeds was served. On June 24, the first case was confirmed as enterohaemorrhagic Escherichia coli (EHEC) O104:H4, the strain identified in the German outbreak that occurred in May 2011. Epidemiological, microbiological, and traceability investigations were conducted to characterize the outbreak, identify the transmission vehicle, and implement appropriate control measures. A cohort study was conducted among all participants in the event, including food handlers. A case was defined as any event participant who presented with either HUS, bloody diarrhea, or an episode of diarrhea between June 8 and 23. Cases were confirmed by isolation of EHEC O104:H4 strains or by O104 serology. Microbiological analyses of clinical and environmental samples were performed, and a traceability investigation was conducted by the European Food Safety Authority. Twenty-four cases, including 22 adults, were identified, and 2 secondary cases were observed within a household. In total, 12 cases were confirmed (11 with serotype O104:H4 and 1 with serogroup O104). The consumption of organic fenugreek seeds was identified as the source of the outbreak, with a significant association with the disease (RR: 5.1 [95% CI: 2.3–11.1]). The seed traceability investigation identified a batch of organic fenugreek (produced in Egypt) common to the outbreak in Germany. Prevention and control measures (recall of the implicated batches) were implemented. This outbreak provided new insights into EHEC infections, notably a longer incubation period and infections in adults who had not previously been targeted by the surveillance system in place in France. (R.A.)