In the Sahel region of Africa, the wild and the cultivated forms of pearl millet, Pennisetum glaucum (L.) R. Br., are sympatric and interfertile and, yet, have remained distinct for millenia. Reproductive barriers are not sufficient to explain this situation. In order to elucidate other possible mechanisms, the two forms have been compared under experimental conditions in the Sahel, for their phenology and reproductive effort. The length of the flowering period of each type was much longer than the average individual flowering period. When the last cultivated plants were finishing flowering 65% of the wild plants were still flowering and 30% were just starting to flower. Thus, the last group was completely isolated from cultivated pearl millet gene flow, in an endogamic reproduction situation. The two forms of pearl millet also differed in the distribution in above-ground biomass among different plant parts, except for the number of seeds per plant. Both phenological behaviour and reproductive effort contribute to the maintenance of distinct forms of wild and cultivated pearl millet.