The US$ is readily accepted on the streets in Belize at a fixed rate of BZ$2.00 to US$1.00.
The official exchange rate at the banks is BZ$1.9825. In addition they also have other charges which they charge on foreign exchange which ends up working out to an actual rate of approximately US$1.00 to BZ$1.95.
Traveller cheques are generally changed at BZ$2.00 to US$1.00
Please be aware that some companies and persons may levy a surcharge ranging from 3% to 5% if you use your credit card in Belize.