Abstract Alcohol use is a significant problem among Latinxs because of the disproportionate burden of physical harms and negative consequences associated with substance use relative to other racial/ethnic groups. Factors associated with the stress related to being an immigrant increase risk for substance use. A theoretically-based cultural adaptation of motivational interviewing (CAMI) that specifically integrated discussion of stressors related to immigration (e.g., discrimination, stigma, social isolation) resulted in significant reductions in harms related to alcohol for those Latinx heavy drinkers with high discrimination compared to standard MI, and reduced anxiety and depressive symptoms one year later compared to MI. Rigorous tests that examine theoretically-informed adaptation of efficacious interventions for addictions are not common, yet are needed to advance implementation science because they address questions that are the basis for successful implementation. The proposed Hybrid Type 1 Effectiveness-Implementation study is an important next step in this line of research, which is to investigate the feasibility of implementing the CAMI intervention in a real-world clinical setting. The key questions are: Would CAMI have positive effects among individuals who use both drugs and alcohol? How do providers view this intervention? We will collaborate with a primary care center that serves a mainly Latinx client population to train their Community Wellness Advocates (CWAs) to deliver the CAMI to patients who are heavy drinkers. We will conduct a concurrent investigation on the process of implementing the CAMI in primary care - a two-arm randomized clinical effectiveness trial will enroll Latinx heavy drinkers (18 years or older) in primary care who use alcohol (and may use other drugs) - and follow them for 12 months after the intervention. Specific Aims are: (1) To examine the impact of CAMI plus an in- person booster session (vs. assessment only) on outcomes: % heavy drinking days, frequency of alcohol- related consequences, depressive/anxiety symptoms, and number of illicit drug use days, using a Hybrid Type 1 Effectiveness-Implementation design and (2) To gather indicators of implementation outcome from multiple stakeholders using a mixed-methods approach. We will follow Curran's framework to evaluate the process of implementation and Proctor's framework to measure implementation outcomes: acceptability, adoption, intervention appropriateness, feasibility, overall cost (i.e., CAMI vs. assessment only), and treatment fidelity. Our study, a first to examine the acceptability of culturally-adapted addiction treatments in primary care settings, will answer essential questions on implementing evidence-based care for Latinxs that can improve health disparities related to substance use. Long term goals are to translate our lessons from the proposed Hybrid study to the broader community to focus on population health for all primary care patients...