Of the 97 described species in the New World genus Ambrysus Stål, the nymphal instars of only one North American species have been described. This work represents the first contribution where the immature instars of a Mexican endemic species are characterized and illustrated, Ambrysus inflatus La Rivers. The descriptions are based on specimens collected in Lago de Chapala, Michoacán, in central Mexico. The main characters to distinguish among instars are related to the overall size and relative length of the mesonotal wing pad compared with the exposed part of the metanotal lateral margin. Some characteristics of the immature instars of Ambrysus inflatus and A. lunatus Usinger are briefly discussed. A comprehensive list with references of the species of Naucoridae for which some or all the immature instars have been described is presented.