At 28 days post-infection autoinfective third-stage larvae (L3a) of Strongyloides stercoralis occurred in jirds infected with 10,000 infective third-stage (L3i). Previously in the jird model of strongyloidiasis, autoinfection had been seen in immunologically immature or immunosuppressed jirds only. The heavily infected jirds described herein had a strong anti-L3i immune response at the same time the living L3a were found in their tissues. This was demonstrated by the rapid killing of L3i in subcutaneously implanted diffusion chambers. No decrease of intestinal motility was observed in these heavily infected jirds, indicating that an increased time for development was not the explanation for the presence of L3a. These larvae were found only in jirds when concomitantly a large number of first-stage larvae (L1) occurred in the intestines. We suggest that the development of L3a in the adult jird model is a rare event and thus, autoinfection occurs only when the intestinal population of L1 is very large, as was the case in the heavily infected jirds. Index Descriptors and Abbreviations. Nematode; Strongyloides stercoralis; Mongolian gerbil; Meriones unguiculatus; autoinfection; larval development; L3i, infective third-stage larva(e); L3i+, tissue migrating third-stage larva(e); L3a, autoinfective third-stage larva(e); L1, first-stage larva(e); PI, post-infection.