Treatment effects of health behavior interventions erode, often with relapse rates greater than 50% within 6 months, perhaps because the constructs that sustain behavior change differ from those that favor behavior acquisition. The overarching aim of this study is to evaluate the efficacy of Mujer Segura Siempre (MSS, Always Healthy Woman), a safer-sex behavior maintenance intervention delivered by means of tailored text messaging. MSS will use theory-based, individually tailored text messages that are designed to maintain the positive effects of an evidence-based counseling intervention (Mujer Ms Segura, R01 DA023877) that was shown to be efficacious in reducing HIV/STI incidence by more than 60% among both drug-using and non- drug-using female sex workers (FSWs) in Tijuana and Ciudad (Cd.) Juarez, Mexico. The content of the text messages will be based on constructs from behavior-maintenance theory. Our specific aims are to: (1) deter- mine if the theory-based MSS text-messaging intervention is associated with the long-term (24-month) mainte- nance of significant reductions in HIV/STI incidence (primary outcome) among drug-using and non-drug-using FSWs in Tijuana and Cd. Juarez relative to a time-equivalent control condition; (2) determine if the MSS intervention is associated with greater reductions in unprotected vaginal and anal sex with male clients, better attendance at regularly scheduled STI screenings, and reduced sharing of drug paraphernalia for FSW-IDUs (secondary outcomes) relative to a time-equivalent control condition; (3) determine if substance use factors (frequency, intensity, and patterns of alcohol and drug use) mediate or moderate the efficacy of the interven- tion, taking into account environmental risk factors (e.g., drugs in the workplace), maintenance constructs (e.g., maintenance self-efficacy), and personal characteristics (e.g., age, sexual abuse, depression); and (4) evalu- ate the cost-effectiveness of MSS compared to the control condition, to Mujer Ms Segura alone (data from previous studies), and to usual care in Mexico. To meet these aims, we will deliver the brief Mujer Ms Segura counseling intervention to 600 HIV-negative FSWs (300 in Tijuana, 300 in Cd. Juarez) who report unsafe sex with at least one client in the previous 6 months, and randomize them to either Group 1 (n=300), the MSS text- messaging maintenance program for 24 months post-counseling (since relapse is most likely to occur within this period) or Group 2 (control condition, n=300), general health text messages for 24 months post-counsel- ing. Both groups will undergo follow-up behavioral assessments and STI testing at 6, 12, 18, and 24 months. This study will advance the use of commonly available information technology to empower high-risk women in lower- to middle-income countries (LMICs) to prolong the benefit of an efficacious HIV prevention intervention. It also addresses the goals of the Office of Research on Women's Health to incorporate gender into the design and development of new technologies for HIV prevention among high-risk women, and to personalize prevention messages that will maintain positive behavioral change.