Descriptions of the last three nymphal instars of the water boatman Sigara (Aphelosigara) tucma Bachmann (Hemiptera: Heteroptera: Corixidae) are provided, for the first time, for specimens collected in northern Argentina; the egg is redescribed. Adults and nymphs were collected in the field. The eggs were obtained by dissection from females. The immature stages were fixed for microscopic examination and illustration and were described with an emphasis on morphometry and chaetotaxy of selected structures. The last three nymphal instars of S. tucma can be easily recognized by the body and head lengths and widths; the number of transverse sulcations of the rostrum; the chaetotaxy of trochanters, protibiotarsus, midlegs, metatibia, and metatarsus; and the grade of development of the wing pads. The eggs of the genus Sigara can be distinguished by the size and the chorionic surface. The chaetotaxy of the mesonotum, metafemur, and metatibia distinguish groups of species belonging to the last nymphal instar of the genus Sigara. A key to the last three nymphal instars of the species of Sigara from Argentina is provided.