Belize is not a major biking destination. In fact, there are only four or so major paved roads in the entire country -- primarily the Northern, Western, Southern and Hummingbird highways. Except for the wild Maya Mountains and Mountain Pine Ridge areas, the country is almost entirely flat. It would be possible to tour Belize's major mainland destinations on a touring bike, but the heat and humidity are often oppressive, and those few highways get plenty of traffic and generally don't have very wide shoulders.
The options are slightly more appealing for mountain bikers and off-track riders, however. Fat-tire explorations are relatively new and underexploited in Belize. Just about every major hotel in the Cayo District and southern zone has mountain bikes for guest use or rental, or can hook you up with a local rental company. These bikes are fine for day trips and nontechnical riding. However, if you plan to do any serious biking you might consider bringing your own rig, as quality mountain bike rentals are still a rarity in Belize. The Mountain Pine Ridge Forest Reserve and the area around it win my vote as the best place for mountain biking in Belize. The scenery's great, with primary and secondary forests, waterfalls, and plenty of trails. Truly hearty bikers can make it all the way to Caracol.
A Tour Operator
Western Spirit Cycling Adventures (tel. 800/845-2453 or 435/259-8732 in the U.S. or Canada; www.westernspirit.com) offers a 6-day guided group tour around the Cayo District for US$1,995 per person. These folks are based out of Moab, Utah, and their off-road credentials are stellar.