The Aerial Phenomena Research Organization (APRO) was an UFO research group started in January 1952 by Jim and Coral Lorenzen, of Sturgeon Bay, Wisconsin. 
The group was based in Tucson, Arizona after 1960. APRO had many state branches, it remained active until late 1988.
APRO stressed scientific field investigations, and had a large staff of consulting Ph.D. scientists. A notable example was Dr. James E. McDonald of the University of Arizona, a well-known atmospheric physicist, and perhaps the leading scientific UFO researcher of his time. Another was Dr. James Harder of the University of California, Berkeley, a civil and hydraulic engineering professor, who acted as director of research from 1969-1982. McDonald and Harder were among six scientists who testified about UFOs before the U.S. House of Representatives Committee on Science and Astronautics on July 29, 1968, when they sponsored a one-day symposium on the subject. 
Astronomer J. Allen Hynek cited APRO and NICAP as the two best civilian UFO groups of their time, consisting largely of sober, serious minded people capable of valuable contributions to the subject. 
In 1969, a sizable portion of APRO's membership elected to form a new group named the "Midwest UFO Network"; this soon expanded and became the Mutual UFO Network (MUFON), still active today.
APRO's credibility took a major blow in the 1970s. Travis Walton claimed to have been abducted by a UFO in Arizona. He was missing for several days, and returned amid a widespread police search and publicity. APRO, in conjunction with the National Enquirer, arranged for a polygraph, which proved inconclusive and, point in fact, had no bearing on the credibility of APRO. The results only measured stress levels, not truth per se. APRO, Walton, and the Enquirer decided to suppress the polygraph results—the examiner was biased, they said, and unprofessional.