The present study compared the efficacy of virtual reality (VR) in virtuo exposure and in vivo exposure in the treatment of spider phobia. Two treatment conditions were compared to a waiting-list condition. A 3-month follow-up evaluation was conducted in order to assess the durability of the treatment effects. Participants were randomly assigned to the treatment groups. A total of 16 participants received the in virtuo treatment, and 16 received the in vivo treatment. The waiting-list condition included 11 participants. Participants received eight 1.5-hour treatment sessions. Efficacy was measured with the Fear of Spiders Questionnaire, the Spider Beliefs Questionnaire (SBQ-F), and a Behavioral Avoidance Test (BAT). In addition, a clinician administered the Structured Interview for DSM-IV to assess DSM-IV's criteria for specific phobia and severity. Clinical and statistically significant improvements were found for both groups. Differences in treatment groups were found on one of five measures of fear: greater improvement on the SBQ-F beliefs subscale was associated with in vivo exposure.