We conducted a line-transect survey via direct observation to estimate the densities of forest duikers in Moukalaba-Doudou National Park, Gabon. Five 2-km line transects were established in different vegetation types, and each transect was walked at 1.0 km/h. We also conducted a ‘quick survey' by walking at c. 2.5 km/h along all transects. A detection function was estimated separately for each species by pooling all the data from the five transects. For Cephalophus silvicultor, all sighting records of the two surveys were used because there was no significant difference in the encounter rates. In total, Philantomba monticola, red duikers (Cephalophus spp.), and C. silvicultor were observed 100, 80, 33 times, respectively. Their individual densities were estimated at 16.43/km² (95% CI: 11.4–23.6), 7.1/km² (5.5–9.2), and 1.63 km² (1.0–2.8), respectively. The encounter rate for P. monticola and C. silvicultor did not vary significantly between the transects, while that of red duikers was higher in the primary forest than in the young secondary or gallery forest. The duiker densities in our study area are among the highest in the African rainforest. Our results indicate that quick surveys may be an effective approach to estimate the densities of rare species.