Technical Abstract: Objectives –(1) Identify the seasonal pattern and risk factors for Escherichia coli O157:H7 in feces in range cattle in California, (2) Determine strain diversity and transition over time using Multi-Locus Variable-Number Tandem Repeat Analysis (MLVA) and Pulsed Field Gel Electrophoresis (PFGE) Samples– Approximately 40 freshly passed fecal samples /month were collected froma cow-calf operation in the California Sierra Foothills over 12 months. Procedures –E.coliO157:H7 was recovered from feces using immunomagnetic separation (IMS) and two selective media, CT-SMAC and NT-Rainbow. Virulence factors were detected with RT-PCR. E.coli O157:H7 positive isolates were subtyped by MLVA and PFGE. Prevalence estimates were calculated and significant risk factors determined by logistic regression. A dendrogram based on the MLVA typing was constructed. Results – Overall prevalence estimate of E. coli O157:H7 was 10.5% (48/456), with prevalence lowest during the winter. Thirty day average temperature before sampling was significantly associated with fecal prevalence of E. coli O157:H7. Nineteen MLVA types and 12 PFGE types were identified. Conclusions and clinical relevance – Our study demonstrated the seasonal pattern of E. coli O157:H7 fecal prevalence in range cattle operation in California. Subtyping using MLVA and PFGE showed a diversity of E. coli O157:H7 strains in this cow-calf operation and turnover of predominant types over time. MLVA and PFGE results were well correlated and MLVA may be a powerful tool in outbreak investigation.