The Tonic Immobility Scale has been only studied in abuse and/or sexual assault survivors, but tonic immobility (TI) also occurs after other types of traumatic events. Consequently, we modified the scale for this purpose and studied its factor structure, internal consistency, and convergent, discriminant and criterion validity in 392 university students, most of whom had suffered one or more traumatic events of various kinds. The Fear subscale of the Modified Tonic Immobility Scale (MTIS) showed very low internal consistency, and must be eliminated or reformulated. Discrimination indices of items 5 and 8 were very low, and these items must be reformulated or removed. After removing the five previous items, the shortened version of the MTIS had a one-factor structure, although model fit was not completely satisfactory. This scale showed acceptable levels of reliability, convergent validity with post-traumatic symptomatology, discriminant validity with measures of depression and anxiety, and criterion validity differentiating between groups that had and had not experienced traumatic events. The psychometric properties of the shortened version of the MTIS should be studied further. It may also be advisable to add new items of TI in order to establish whether this is a construct with more than one dimension.