Upon its initial theatrical release, the film had mixed reviews. In a generally positive review, Jay Cocks of "Time" saw the film as "good, eerie entertainment, with interludes of such haunted visual intensity that it becomes, at its best, a nightmare incarnate". In a negative review, "Variety" wrote that this ecological monster film "didn't get the bugs worked out before release". "Time Out London" wrote that the special effects take priority over the ideas. A. H. Weiler of "The New York Times" wrote, "For all of its good, scientific and human intentions, 'Phase IV' cries for a Phase V of fuller explanations."