Ecology may be conveniently defined as the science which deals with the relationship of an organism to and with its environment. My visualization of this concept is one of a dynamic inter-relation. Perhaps we could borrow a phrase from the physical chemist and refer to this as a dynamic fluctuating equilibrium. This would include the effects of the environment on the organism in all aspects of its life, on its metabolism, on its reproduction, its continuing evolution or survival as an individual, a clone, or a species. It would include the response of the organism, individually and as a group, to a changing environment and, equally true, the response of the environment to the changing conditions imposed by the organism. We are accustomed to apply the term ecology to the relation between free-living organisms and their environment. One speaks readily enough of the ecology of mosquitoes or of the ecology of the free-living hookworm larvae. Should we not in like manner consider the ecology of the malaria parasite in the mosquito or the ecological evolution of the hookworm as it moves from its free-living through its several environments in the mammalian host on its way to the small intestine? Probably there would be rather general acceptance of a statement that the ecology of a parasite, particularly one which requires two or three different species of hosts in its ontogeny, is more complex than the ecology of a free-living organism. Yet I wonder if this is really true. Is the septic tank or the pond in which the ciliate lives any less complex than the alimentary canal in which the flagellate lives? Do the two hosts of the malaria parasite offer any wider variety of environmental factors than that which the salmon encounters in its migration from its many haunts in the ocean depths to its spawning beds above the rapids of the river? It is not my purpose to dwell on such comparisons or even attempt to answer the questions I pose. I raise these questions merely as the stepping stones for my more immediate question-do we adequately consider the parasite in its ecological relations? Have we adequately considered the ecological significance of the many diverse facets of information which have been provided by the comparative taxonomist, the ontogenist, the physiologist, the enzyme chemist, the immunologist, the immunochemist, the pathologist, the experimental nutritionist, the epidemiologist, and the clinician? When I see, in a modern day textbook, the epidemiology of hookworm disease confined to the simple statement that eggs are passed in the feces, development takes place in sandy soil, and infection is by way of skin penetration, I wonder. When I further note in contrast that paragraphs * Address of the retiring President of the American Society of Parasitologists, 32nd Annual