The armyworm, Pseudaletia unipuncta (Haworth), is a serious pest to the Azores’s pastures. In laboratory bioassays we tested the susceptibility of this insect to entomopathogenic nematodes isolated in Azores: Steinernema carpocapsae Az20, Az150, and A48 strains, S. glaseri Az26 strain and Heterorhabditis bacteriophora Az33 strain. The A48, Az20, and Az150 strains caused parasitism rates of 96.6, 90, and 53.3%, and mortality rates of 63.3, 46.6, and 23.3%, respectively, to sixth instar. The Az33 strain caused a parasitism rate of 73.3% and a mortality rate of 40%; whereas, the Az26 strain caused a parasitism rate of 40% and no mortality. A linear response dose—parasitism with a positive regression (r² = 0.993) was observed in insects exposed to S. carpocapsae Az150 strain. Positive regressions were also observed between mortality and dose rate for S. carpocapsae A48 (r² = 0.980), Az20 (r² = 0.956), and Az150 (r² = 0.963) strains, and H. bacteriophora Az33 strain (r² = 0.999). Fourth instars were the most susceptible to the A48 strain, followed by the fifth instars, while the sixth instars were the less susceptible, with LD₅₀ values of 26.2, 62.8, and 320.7 infective juveniles, respectively. The lethal time for each of the tested instars was 32.3, 35.5, and 49.2 h, respectively. The invasion rate was 33.5, 28.2, and 40.8 nematodes per treated larvae in the fourth, fifth, and sixth instars, respectively.