 As a master's degree candidate at the University of Illinois, he organized and led (1938–1941) four multi-disciplinary biological expeditions into the mountain and desert portions of western and southwestern Mexico.These resulted in the amassing of large, scientifically valuable collections of animals and plants.During World War II, Hoogstraal was assigned to the U.S. Army 19th Medical General Laboratory, near Hollandia, Dutch New Guinea, where in 1945 he and Willard V. King engaged in a massive taxonomic study of the mosquitoes of that area.