Although some research suggests that misinformation provided by a co-witness could be more influential than that obtained from other sources, most of this research has compared the effect of co-witness information against non-social forms of misinformation only. To better understand the influence of co-witnesses we compared the influence of co-witness misinformation with the influence of misinformation provided by an interviewer. Across two experiments using the MORI paradigm we found no evidence that a co-witness is particularly influential relative to another social source of post-event misinformation. In fact, the source of the misinformation delivered by our interviewer was less likely to be correctly recalled than the source of the misinformation delivered by a co-witness. There was some evidence that misinformation delivered by both a co-witness and an interviewer has a stronger effect on witnesses' accuracy and confidence than misinformation obtained from either source alone. Finally, our results suggest that the opportunity to provide an early individual memory account might protect against the effect of subsequently-encountered co-witness misinformation. These results have important implications for the way that criminal investigations are conducted.