Phase IV is a 1974 science-fiction horror film directed by graphic designer and filmmaker Saul Bass, and written by Mayo Simon, inspired by H. G. Wells's 1905 short story "Empire of the Ants".

It has since gained a cult following, due to TV airings beginning in 1975 and also being shown on Mystery Science Theater 3000 during the KTMA era.

After a spectacular and mysterious cosmic event, ants of different species undergo rapid evolution, develop a cross-species hive mind, and build seven strange towers with geometrically perfect designs in the Arizona desert.

Scientists James R. Lesko and Ernest D. Hubbs set up a computerized lab in a sealed dome located in an area of significant ant activity in Arizona.

[5] The idea for Phase IV was apparently hatched over cocktails in 1971, when Peter Bart at Paramount had dinner with Raul Radin and asked him what's cooking.

According to the book Future Tense, "Bass originally filmed a spectacular, surreal montage lasting four minutes, showing what life would be like on the 'new' Earth, but this was cut by the distributor.

[9] This recovered montage ending, along with a brand new 35mm print of the theatrical version, premiered at the Alamo Drafthouse in Austin, Texas, as a part of a full day of films by Saul Bass in December 2012, before being shown at select arthouse theaters in other cities.

A making-of documentary, a commentary track with film historian Matthew Asprey Gear and an interview with composer Brian Gascoigne are included as bonus features.

[15] Brian Gascoigne was the chief composer and Stomu Yamashta was responsible for the music in the final montage sequence, which was cut from the theatrical release.

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".

[19] 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.

[22] Bill Gibron of PopMatters rated it 7/10 stars and wrote that "for every hackneyed hole-punch moment there’s an engaging scope enhanced by the film’s visual wonders".

[23] David Cornelius of DVD Talk rated it 4.5/5 stars and wrote, "Watch it late at night with the lights out, and you'll get plenty freaked.

"[24] Phase IV won the 1975 Grand Prix Award at the International Festival of Science Fiction Films in Trieste, Italy.

The film predates by two years the first modern reports of crop circles in the United Kingdom and it has been cited as a possible inspiration or influence on the pranksters who started this phenomenon.