Rats were infected with known numbers of trichina larvae, and the location of the worms in the intestine was ascertained for different periods. In all cases, from the beginning of the infection to the elimination of adults, it was demonstrated that the number recovered was greatest from the anterior portion of the samll intestine, and steadily declined posteriorly. Immediately after ingestion the larvae penetrated the mucosa between the villi and developed to maturity. However, between 20 and 24 hrs. many appeared again in the intestinal lumen, perhaps for the purpose of copulation. Subsequently both sexes repenetrated the mucosa with many touching the muscularis, causing great destruction of glandular tissue. It has been generally believed that the sex ratio was 1 to 1, but throughout this work the calculated ratios revealed that during the main period of reproduction the females outnumbered the males approx. 2 to 1. After the 9th day, the number of females decreased more rapidly than that of males so that the sex ratio was nearly 11 at end of the intestinal phase of Trichinella infection.