AY, CUCARACHA! SCARLETT JOHANSSON screamed bloody murder after she accidentally locked herself inside a bathroom – then encountered a creepy-crawly creature while filming the new “Captain America” in a shabby old building in downtown LA.

“Scarlett was just finishing up, but as she went to wash her hands, a huge cockroach fell from the ceiling, barely missing her, and plopped right into the sink bowl,” said my secret source, Onda Settalot.

“Like many women, Scarlett’s deathly afraid of the nasty bugs, and she let out a howl that sent shock waves through the building! Crew people came running as Scarlett kept shrieking that she was trapped inside because the door had somehow jammed!”

The crew couldn’t budge the door, but one guy finally got Scarlett to calm down – then told her to take off her shoe, use it as a hammer and pound on the lock.

Said Onda: “After a few tries, Scarlett finally managed to whack the latch open, and she burst out of that toilet like a bat out of hell. Then she told producers she’d never enter it again, and demanded a private, roach-free porta-potty!”