We describe the second and third instars of Hydroporus zimmermanni J. Müller, 1926 on the basis of material collected from the type locality in Petelinsko Jezero, San Pietro del Carso (now Pivka) in present-day Slovenia. We also provide notes on its ecology and a key to the known third instars of Hydroporus Clairville, 1806, which have been described in much detail elsewhere. The larva of H. zimmermanni resembles those of other known species of the genus, but has a more elongate urogomphus.