Visitors to the Bronx Zoo were greeted with locked doors when they tried to enter the reptile exhibit this past weekend, and with good reason: a venomous snake was on the loose.

“The World of Reptiles is closed today,” a sign explaining the closing said. “Staff observed an adolescent Egyptian cobra missing from an off-exhibit enclosure on Friday.”

The Egyptian cobra, a favorite of snake charmers — and probably the asp whose venom Cleopatra used to commit suicide — is a dark snake with a narrow hood, and grows up to two yards in length. (The missing animal was only 20 inches, a zoo employee said.) Native to Africa and the Arabian Peninsula, it usually preys on toads and birds, not humans, but zookeepers notified the public in an abundance of caution. The snake’s toxins can cause respiratory failure.

The director of the zoo expressed confidence that the snake was still in the reptile house and said the snake would probably avoid open areas. “To understand the situation, you have to understand snakes,” Jim Breheny, the director, said in a written statement. “Upon leaving its enclosure, the snake would feel vulnerable and seek out a place to hide and feel safe. When the snake gets hungry or thirsty, it will start to move around the building. Once that happens, it will be our best opportunity to recover it.”