The aim of the current study was to investigate the susceptibility to the insecticide deltamethrin and the expression of resistance to this insecticide in developing eggs and neonate nymphs of Triatoma infestans from two areas of Argentina (Campo Largo) and Bolivia (Entre Ríos), where resistance to this pyrethroid is suspected. Both nymphal populations showed resistance to deltamethrin, with lower resistance ratio for Entre Ríos (173×) than Campo Largo (1108×). Efficacy of deltamethrin on 4-, 7-, and 12-d-old eggs for both field populations were significantly lower than efficacy on eggs of the susceptible strain. This is the first documented evidence of the expression of pyrethroid resistance during the embryonic development of Chagas Disease vectors.