D U URING the last quarter of a century a great deal of attention has been given to the group of parasites known as the hookworms, which infest man and certain animals. The chief reason for this attention has been the importance of these parasites in the field of public health, but out of the work have come many observations which are of interest from a purely biological point of view. Although these contributions have dealt with both the free living and the parasitic stages, it is with the latter that this paper will primarily be concerned. The chief purpose is to summarize certain experimental studies on the biology of the species of hookworms parasitic in cats and dogs. Although an attempt will be made to mention representative papers from the literature of this field, emphasis will be laid on the work of a particular group done under the general direction of Dr. W. W. Cort and supported by a grant from the International Health Division of the Rockefeller Foundation. These species found in cats and dogs furnish the most convenient material for experimental work. Although these forms are related closely to those found in man, conclusions from the experiments should be applied to the human hookworm only with the greatest caution.