Field collections and laboratory observations document for the first time nymphal Stagmomantis Carolina (Johannson) (Mantodea: Mantidae) as a host of Masiphya confusa Aldrich (Diptera: Tachinidae). In Washington County, MS, field parasitization rates of nymphal mantids in old-field sites exceeded 80% in late summer and were associated with a delay in maturation of parasitized nymphs. Laboratory rearings and dissections indicated that early-stage parasites had entered at the lateral mesothorax and were free-moving in the host thorax, with late-stage larvae attached to the inner wall of the fifth abdominal segment. A sclerotized airhole was present at the attachment site and exit holes for the mature larvae were in the area of the second abdominal segment. When attached, the parasite was encased in a sac and a sclerotized respiratory funnel was formed. After parasite emergence, the host typically died within 24 h, although some host individuals survived for up to 29 d and some individuals fed and partially molted after parasite emergence under ambient conditions. Pupation of parasites under ambient conditions occurred in the soil in September and October, with subsequent emergence in the following May.