A new species of Pseudacteon phorid fly, Pseudacteon calderensis (Diptera: Phoridae), is described from females attacking worker ants of Solenopsis interrupta Santschi in Salta and Jujuy provinces, northwestern Argentina. P. calderensis differs from almost all other South American Pseudacteon species by the structure of the female ovipositor. This new species is morphologically similar to Pseudacteon borgmeieri Schmitz, but it differs slightly in the shape of the apex of the ovipositor and differs strongly because of the absence of large and stout ventral hairs under the ovipositor.