In order to know the population dynamics of the fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), and its parasitoids in northwestern Argentina, larvae were weekly collected at two different agrological regions (Tafí Viejo, and Vipos) over four years. The relationship between larval and parasitoid populations, climatologic factors, percent of infested plants, parasitoid relative importance index, abundance of the parasitoids, and percent parasitism were estimated. FAW attacked cornfields when the plants achieved V1 and V2 stages. Temperature and rainfall were the climatologic factors that significantly affected pest density, and temperature affected the parasitoid abundance as well. The FAW parasitoids collected were Campoletis grioti (Blanchard), Chelonus insularis (Cresson), Ophion sp. and Archytas spp. (possibly marmoratus and/or incertus). The average parasitism percentage was 39.4% and 15% in T. Viejo and Vipos, respectively. Parasitoid abundance in both regions was similar, but diversity was different possibly relating to the native surrounding vegetation in Vipos. This is the first report of population dynamics of the fall armyworm and its parasitoids in northwestern Argentina.