Additional information is given on the erythrocytic stages of Plasmodium petersi (Poirriez, Baccam, Dei-Cas, Brogan et Landau, 1933), which was found in a Cercocebus albigena monkey from the Central-African Republic. The first colour pictures of P. petersi are presented. In 60% of young trophozoites, the vacuole is divided into two or three parts by thin cytoplasmic streaks. In young trophozoites and almost mature schizonts, 80% of nuclei are oval or kidney-shaped; they are two-coloured; measurement of their surface area shows that it is about twice that of the nuclei of P. gonderi at the same stages. Studies using polarised light show that most of the pigment granules are elongated (spindle-shaped) and found at the periphery of old trophozoites and schizonts. P. petersi can easily be distinguished from P. gonderi and P. georgesi, the two other species found so far in Cercocebus monkeys, which are regarded as the African equivalents of the Asian macaques.