Tonic immobility (TI) is an involuntary component of the fear response that is characterized by freezing or immobility in situations involving extreme fear coupled with physical restraint. The present investigation evaluated the factor structure of the Tonic Immobility Scale (TIS; Forsyth, J. P., Marx, B., Fusé, T. M. K., Heidt, J., & Gallup, G. G., Jr. (2000). The Tonic Immobility Scale. Albany, NY: Authors)--a newly developed measure to assess components of TI in female sexual assault survivors. Study 1 (N=88) consisted of an Exploratory Factor Analysis of sexual assault survivors' responses on the TIS, whereas Study 2 (N=191) involved a Confirmatory Factor Analysis (CFA) with a second independent sample of female sexual assault survivors. Findings from both studies suggest that the TIS is comprised of two independent factors: physical immobility and fear. Findings are discussed in terms of the theoretical and practical implications of the factor solution obtained, particularly with regard to evaluating TI in adult survivors of sexual assault.