The Coast Guard said the collision occurred in the afternoon on a dazzlingly sunny day. Investigators had no immediate word on what might have caused the accident or the number of people aboard the two planes.
Fire and lifeguard boats and rescue divers swarmed the area about two miles outside the entrance to the harbor, where water depths were 80 feet to 90 feet.
Debris that included the full tail number for one plane and partial number of the other was found, Los Angeles County lifeguard Capt. Ken Haskett said.
The nearest harbor entrance was closed to traffic while the search continued.
The crash site was a quarter-mile south of the Angels Gate light, a lighthouse at the San Pedro Breakwater that is on the National Register of Historic Places. The area is popular for flight students, and there were many planes in the crystal-clear skies at the time of the accident.
Pilots communicate at two different radio frequencies - one for above 2,000 feet and the other below, said Reed Novisoff, chief pilot at Pacific Air Flight School.
"People are very diligent about reporting their positions," Novisoff said. "It's very safe out there."
Nonetheless, Friday's midair collision was not the first.
In 2001, four people died when two Cessna airplanes carrying instructors and students collided 1,000 feet above the harbor.
In 1986, two small planes flown by students collided. But the aircraft managed to return to their airports and the four people on board escaped injury.
Richard Garnett, chief flight instructor with the Long Beach Flying Club, said the pilots practice in an area that is 10 to 20 square miles and at altitudes ranging from 1,000 to 4,000 feet. On a typical day, there will be three or four planes in the air at the same time.
"So with the amount of activity, actually, I think we've been fortunate," he said. "We are really diligent. I don't know why, what happened in this situation."
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