Claustrophobia, a fear of enclosed spaces, is thought of as being a unitary phenomenon. However, different subtypes of claustrophobia may exist. Some claustrophobics may be more similar to individuals with Panic Disorder with Agoraphobia in terms of situations feared, cognitions and symptoms whereas others might be similar to simple phobics with a specific fear of enclosed spaces. The purpose of this study was to investigate whether such subtypes exist. The Claustrophobia Situations Questionnaire (CSQ) and the Claustrophobia General Cognitions Questionnaire (CGCQ) were developed and exploratory factor analyses were performed on these scales. Two-factor solutions for both the anxiety and avoidance ratings on the CSQ were obtained accounting for 40.8% and 33.8% of the variance, respectively. Two subscales were created from each two-factor solution. A three-factor solution was obtained for the CGCQ accounting for 53.6% of the variance from which three subscales were created. Differential validity of the scales and their utility in identifying claustrophobic subtypes was assessed. Self-report measures and physiological response to a hyperventilation challenge were used to validate claustrophobic subtypes. A significant main effect for "Avoidance of Crowds" was found when using heart rate change and post heart rate as dependent measures. Specifically, subjects high on the "Avoidance of Crowds" subscale demonstrated greater heart rate change and post heart rate than subjects low on the "Avoidance of Crowds" subscale. This suggests subjects avoidant of panic-like situations had a greater physiological reaction to the hyperventilation challenge, a task considered to be problematic for panic disordered individuals. Therefore, the results generally suggest the existence of claustrophobic subtypes. The present study was the first to compare subjects differentiated on the basis of claustrophobic subtypes in terms of their physiological response to a hyperventilation challenge. This study both supported and extended past research by developing questionnaires (the CSQ and CGCQ) capable of identifying different claustrophobic situational and cognitive factors. / Master of Science