SEN viruses (SENVs) are newly discovered bloodborne viruses that may play a role in liver disease. SENV strain prevalence was examined in a race/ethnicity-stratified sample of 531 injection drug users (IDUs) from the San Francisco Bay area. Weighted prevalences were as follows: SENV-A, 45.7%; SENV-C/H, 35.6%; and SENV-D, 10.3%. Infection was associated with a longer duration of injection drug use. SENV-A was more common in black subjects (adjusted odds ratio [OR(a)], 4.37; 95% confidence interval [CI], 2.65-7.21) and Hispanic subjects (OR(a), 2.30; 95% CI, 1.38-3.85) than in white and non-Hispanic subjects, and the pattern was similar for SENV-C/H. For SENV-D, prevalence was similar in black and white subjects, but lower in Hispanic subjects; infection was less common among women than men (OR(a), 0.32; 95% CI, 0.15-0.71) and more common among men with at least 1 recent male sex partner than among heterosexual men (OR(a), 7.05; 95% CI, 2.62-18.95). SENV strains are common among San Francisco Bay area IDUs, and prevalence varies demographically within this group.