Middle Jurassic strata are naturally exposed around the village called La Baume, near Castellane (Alpes de Haute Provence, SE France). We have realized both a detailed log and a bed-by-bed sampling for ammonite biostratigraphy in a 68 metres thick succession of subpelagic marls and limestones ("Calcaires A Zoophycos") that spans the Middle Jurassic from uppermost Aalenian to lowermost Bathonian. The subpelagic succession can be roughly subdivided into three members. Ammonites from the Upper Aalenian Concavum Zone, all Lower Bajocian Zones (Discites Zone, Laeviuscula Zone including Ovale Subzone, Humpriesianum Zone), one Upper Bajocian Zone (Parkinsoni Zone) and probably one lower Bathonian Zone (Zigzag Zone) were found. A major gap of both the Niortense Zone and the Garantiana Zone, which was not previously described, was detected at the boundary between members 2 and 3. The main palaeontological interest of the ammonite fauna from La Baume is the richness and diversity of the family Sonniniidae, which is the subject of a systematic study and figurated along with some biostratigraphically significant forms. Biostratigraphical results and open problems are discussed.