In addition to an exaggerated startle reflex, the most commonly reported symptoms of post- traumatic stress disorder (PTSD) are intense physiological reactivity and psychological distress, respectively, on exposure to cues that symbolize or resemble an aspect of the traumatic event. While the presence of an objectively measured exaggerated startle reflex in PTSD is firmly established, questions regarding when it is exhibited and what it tells us about the disorder remain unanswered. Furthermore, the data on emotion-modulated startle reactivity in PTSD are especially unclear and need further exploration. This is in large part due to a surprising paucity of data in the literature, as well as the interrelated problems of within-category heterogeneity and diagnostic comorbidity. Given that emotion-modulated startle is such a well-validated measure of emotional processing and defensive system reactivity, the lack of knowledge on this startle blink parameter is striking. The current study explored emotion-modulated startle reactivity in PTSD by investigating startle responses to pleasant, neutral, unpleasant, and trauma-related picture stimuli. Additional information regarding emotional dysregulations in PTSD was obtained through analyses of facial EMG, skin conductance, and heart rate responses. Both categorical (i.e., PTSD vs no-PTSD) and dimensional (i.e., specific symptom cluster) analyses were conducted in order to develop psychophysiological models and measures of the emotional dysregulations in PTSD. Contrary to expectations, categorical comparisons of individuals with and without PTSD did not yield a clear pattern of fruitful differences on any psychophysiological measure other than heart rate. In categorical comparisons, there was no strong indication for a particular physiological model for emotional dysregulations in PTSD and present results provide very little support for the presence of an objectively-measured exaggerated startle response in PTSD. However, significant relationships between most specific symptom clusters and the startle blink response were observed. The same was true for subjective ratings of the affective valence and arousal of the picture stimuli. Unexpectedly, the most consistent effects were for diminished emotion modulation to pleasant stimuli rather than exaggerated responding to unpleasant or trauma-relevant stimuli. Current results suggest that PTSD is not defined by an abnormality in fear and that an underappreciated and central aspect of PTSD is a limited capacity for positive affect. Implications regarding the coherence of the construct of PTSD as defined in DSM-IV are discussed.