The adults of Amblyomma dubitatum Neumann, 1899, originally reported as -collected in southern Spain- are redescribed from the engorged female type and several flat adults collected in Uruguay and Brazil. As stated elsewhere, A. cooperi Nuttall and Warburton, 1908, a well known parasite of Hydrochaeris hydrochaeris (Linnaeus, 1766) [=Hydrochoerus hydrochaeris (L.)] in the Neotropical region, is a junior synonym of A. dubitatum. The nymph of A. dubitatum is described from semiengorged specimens collected on H. hydrochaeris. Immature stages of A. triste Koch, 1844, are described from specimens originating from engorged females and engorged larvae, respectively. A. triste is included in the maculatum group. It is easily distinguishable from other species in this group because of a unique combination of shape and size of auriculae, as well as the relative length of coxal spurs.