OBJECTIVE: This study systematically evaluated the psychological reactions of a nonclinical population to the October 1989 earthquake in the San Francisco Bay Area. METHOD: A representative group of about 100 graduate students from two different institutions in the Bay Area volunteered to participate in the study. Within 1 week of the earthquake, the authors administered a checklist of anxiety and dissociative symptoms to the subjects, and 4 months later they conducted a follow-up study with the same checklist. RESULTS: The participants reported significantly greater numbers and frequency of dissociative symptoms, including derealization and depersonalization, distortions of time, and alterations in cognition, memory and somatic sensations, during or shortly after the earthquake than after 4 months. To a lesser degree they also reported significantly more nonsomatic anxiety symptoms and Schneider's first-rank symptoms at the earlier testing time. CONCLUSIONS: These results suggest that among nonclinical populations, extreme distress may significantly increase the prevalence and severity of transient dissociative phenomena and anxiety. They provide further evidence of the role that dissociation plays in the response to trauma and are of considerable clinical and theoretical importance in view of the lifetime prevalence of traumatic experiences in the general population.