Summary. The White-bearded Greenbul Criniger ndussumensis of the Lower Guinea and Congo Basin forests is so similar in appearance to the sympatric Red-tailed Greenbul C. calurus that separating them in the field or hand is difficult. C. ndussumensis is, however, more closely related to the Yellow-bearded Greenbul C. olivaceus of Upper Guinea, from which it differs in aspects of plumage, but resembles closely in song and in morphometrics, including sharing a narrower bill. C. olivaceus shows distinctive scansorial foraging behaviour, gleaning food from the bark of trunks and branches, something which is unrecorded in sympatric populations of C. calurus. C. olivaceus is shown here to have modifications to the structure of its feet, including long, strongly curved claws, inferred to be related to its scansorial behaviour. Measurements also show that C. ndussumensis exactly resembles C. olivaceus in foot morphology and in bill shape, and that they differ significantly in both characters from C. calurus. Thus, a combination of long claws and narrow bills characterise C. ndussumensis whilst short claws and wide bills are diagnostic of C. calurus, features which enable unambiguous determination in the hand. The differences in foot morphology imply that the only species capable of bark-clinging behaviour in Lower Guinea and the Congo Basin is C. ndussumensis and that C. calurus cannot do so, contra numerous literature reports. Other characters, morphological, behavioural and ecological, by which the two species differ are reviewed and assessed, and attention is drawn to a distinctive call, apparently unique to C. ndussumensis. In the field the most reliable means of separation are behaviour, song, call and the colour of the undertail-coverts, pale cinnamon in C. ndussumensis, yellow in C. calurus, although this latter character is not wholly reliable. No differences in the colour of the lores were found despite statements to the contrary. This study also refutes reports in the literature of intergrades or hybrids between C. ndussumensis and C. calurus in parts of eastern Congo-Kinshasa.