The paper deals with some additions and corrections of preceding works on Coleoptera Hydradephaga from Gabon, and a rectification of "A revision and synopsis of the African species of the genus Copelatus Erichson, 1832. The group erichsonii, subgroup atrosulcatus (Coleoptera Dytiscidae)" (Bilardo & Rocchi, 2015). Three new species of diving beetles are described, namely: Hydrovatus batekensis n. sp. of the villiersi group, close to H. niger Gschwendtner, 1938 and Hydrovatus bistroemi n. sp. of the oblongipennis group, close to H. lintrarius Guignot, 1958 and H. omentatus Guignot, 1950; both new species are principally distinguishable by the shape of aedeagus. The third new species is Copelatus terzanii n. sp. of the irinus group (subgroup assimilis) close to C. efoutensis Bilardo & Rocchi, 1995 and C. curtistriatus Bilardo & Rocchi, 1995; also that new species is distinguishable above all by the shape of aedeagus. The male of Yola marginata Biström, 1983, unknown up to now, is described. The second record from Gabon of Copelatus mvoungensis Bilardo & Rocchi, 2004 is reported. The following synonymies are proposed: Copelatus apuzzoi Bilardo & Rocchi, 1999=C. pederzanii Bilardo & Rocchi, 1995 and Copelatus assimilis incristatus Bilardo & Rocchi, 2002=Copelatus assimilis Régimbart, 1895. The paper Bilardo & Rocchi (2015) contains a misleading phrase concerning the subdivision of the subgroups atrosulcatus/vigintistriatus; it was therefore necessary to replace the ambiguous phrase with a correct one. The rectified phrase is reported herewith in French and in English.