When it opened its gates in 1899, 843 animals were exhibited in small cages and enclosures. Only the facade of the original reptile house remains. Today the 265-acre spread is the world's largest urban zoo. The zoo's more than 4,500 animals, representing more than 600 species, mostly live in outdoor settings designed to re-create their habitats. You're often separated from them by no more than a moat. Planning your trip is important, as it's impossible to see everything here in one day. Try to visit the most popular exhibits, such as Congo Gorilla Forest, early to avoid lines later in the day. Take advantage of the Zoo Shuttle (COST: $2) that circles the entire park. You can get back on as many times as you wish. The Skyfari (COST: $2) is an aerial tram that whisks you above the trees. Remember that in winter the outdoor exhibitions have fewer animals on view.

One of the most fascinating exhibits is the Congo Gorilla Forest (COST: $3), a 6½-acre re-creation of an African rain forest with treetop lookouts, wooded pathways, lush greenery, and 300 animals -- including red-river hogs, black-and-white colobus monkeys, and two troops of lowland gorillas. Jungle World re-creates a tropical rain forest and mangrove swamp filled with white-cheeked gibbons, tree kangaroos, black leopards, and other exotic critters. In Wild Asia (COST: $3), open from April to October, Asian elephants roam free on nearly 40 acres of open meadows and dark forests. Siberian tigers roam through Tiger Mountain, a setting that re-creates the Amur Valley, which borders China and Russia.

The World of Darkness is a windowless building that provides a rare glimpse into the nightlife of such nocturnal creatures as fruit-eating bats and flying foxes. There's even a creepy glow-in-the-dark scorpion. From late May to September, 1,000 butterflies and moths of 35 species dazzle visitors in the Butterfly Zone (COST: $2). The Children's Zoo (COST: $2), open from April to October, has many hands-on learning activities, as well as a large petting zoo. Youngsters can see the world from an animal's perspective by crawling through a prairie dog tunnel and trying on a turtle's shell for size.

The easiest way to get to the zoo is to take the Liberty Lines (PHONE: 718/652-8400; www.libertylines.com) BxM11 express bus from Manhattan. You can also take the No. 2 or 5 subway to Pelham Parkway and walk three blocks west to the zoo's Bronx Parkway entrance. COST: $11; free Wed., donation suggested; extra charge for some exhibits; parking $7. Subway: 2, 5 to Pelham Pkwy.; BxM11 express bus to zoo entrance.