1. Compared with the mammalian intestine, uterus and blood serum, the corresponding organs and perienteric fluid of Ascaris lumbricoides were found to have a high phosphorus content. The muscle of the parasite, however, contained little more than half the phosphorus found in the mammalian muscle.

2. A. lumbricoides took up inorganic ortho-phosphate largely or exclusively through the alimentary tract. Small amounts were taken up by the cuticle and the lateral line system, but did not appear to pass on to other tissues.

3. Nippostrongylus muris in the intestine of the rat took up radioactive phosphorus which had been injected into the host intramuscularly. When the host was dosed per os the radioactivity appeared in the parasites at a lower concentration than in the host's small intestine.

4. Ascaridia galli failed to show any radio-activity when host birds were injected intravenously with inorganic orthophosphate containing P32. A sudden rise and fall in the radioactivity of the parasites were found when host birds were dosed per os.

5. It is suggested that nematode parasites have a high demand for phosphate which they take up largely per os. Ascaridia galli feeds rapidly, but with poor phosphate absorption, on host gut contents. Nippostrongylus muris feeds on host tissues. It is suggested that the different feeding habits of N. muris and Ascaridia galli may have some relation to the efficacy of anthelmintics against these parasites.

The authors are indebted to Dr M. Kamen and Dr E. Reinhard of Washington University, St Louis, for a gift of radioactive phosphorus. Thanks are also due to Mr E. Parrish and Mr L. Turk of the McMaster Laboratory for their capable assistance.