In December 1910 the Zoological Garden of Rotterdam obtained, from a dealer, an interesting semi-adult female of a Cercopithecus, said to be imported from the Upper Congo, and belonging to the Mona-group as defined by Mr. R. I. Pocock in Proc. Zool. Soc. 1907, p. 708. This specimen belongs, as far as I am aware, to a hitherto undescribed species, which I propose to name Cercopithecus petronellae.