Résumé : In 1998, researchers in Burkina Faso enrolled 300 women more or less involved in commercial sex work in an open cohort to determine whether adequate management of their sexually transmitted infections and exposure to well-designed, well-delivered, and plentiful communication for behaviour change (CBC) might reduce their vulnerability to HIV. In 2000, they observed that the non-professional sex workers (occasional or clandestine sex workers) were more difficult to reach, to mobilize and to keep involved in the project's different activities. This group was also infected at the same or higher rates than professional sex workers because they did not use condoms routinely. To accomplish the project objectives, they therefore chose to recruit more non-professional sex workers in the new cohort of 700 women. This social-anthropological study was conducted to help them to enrol young clandestine sex workers. The overall objective of this study was to understand the life of this category of sex workers and to identify strategic actors to reach them. Using a qualitative method, social anthropologists reviewed literature, identified and geo-referenced all local places suitable to encountering these women, obtained life stories from some of them and interviewed key informants and participants in the field. The results showed that in Bobo-Dioulasso (Burkina Faso): - most young women who are clandestine sex workers are Burkinabé, and girls entering the sex trade are increasingly young and increasingly uneducated; - most of them come from families with low capital (financial, cultural, or social). The parents' socioeconomic status (contextual poverty) results in unmet financial needs, which in turn exposes them to starting work early, including commercial sex work; - of all the income-generating activities available to unskilled young girls, commercial sex work is one of the most profitable and easily accessible; - in the three-fold context of an HIV epidemic, poverty, and unemployment, clandestine commercial sex work is a rational action, insofar as condom use reduces the risk of HIV infection, "clandestinity" reduces the risk of social stigma, and earnings increase financial capital; - girls are coopted into sex work through an initiation process and the initiator explains to the initiate how sex workers think, act, and live, as well as the rules of the trade; - young clandestine commercial sex workers use various strategies to do their work in secret, unidentified, by changing the time, place, period, district, city or country of their work; - young clandestine commercial sex workers maintain friendly relations with men or boys in but have no or conflictual relationships with women and girls. Thus, only other participants in this trade, peer counsellors, and room renters can serve as strategic actors to reach, mobilize and keep these young girls in HIV programmes. Social anthropologists have concluded that one problem in the fight against official or professional commercial sex work is the development of clandestine commercial sex work, which is more dangerous, firstly for its practitioners, who are harder to reach by messages about HIV and thus do not change their behaviour, secondly, for their sexual partners who do not use condoms systematically, and finally for society as a whole, to the extent that social actors are embedded in an informal network, more or less extensive, of sexual partners.