Disparities related to socioeconomic status (SES) have not been examined in the quality of behavioral health care within Medicare Advantage (MA) plans. This study used mixed-effects regression models to examine SES-related disparities within MA plans on HEDIS measures of quality of four aspects of behavioral health care: management of antidepressant medication, initiation of substance use treatment, follow-up after hospitalization for a mental illness, and avoidance of drug-disease interactions. The model includes random effects for plans and fixed effects for low SES, defined by Medicaid enrollment or receipt of a low-income supplement. Within MA plans, quality measure performance for low-SES beneficiaries is higher for substance use treatment initiation (OR=1.1, 95% CI 1.08-1.12) and lower for all other measures (ORs 0.68-0.95), compared to high-SES beneficiaries. However, SES-related differences are weaker or reversed in racial/ethnic minority groups. SES-related disparities in behavioral health care quality are present in MA plans and differ by race/ethnicity in ways that should be considered in the design of quality reporting systems.