http://belizebus.wordpress.com/2012/07/27/crossing-the-border-from-corozal-to-chetumal/

9. TRANSPORTATION BETWEEN BELIZE AND GUATEMALA


1. Fuente del Norte, Linea Dorada, and San Juan Travel Services (Guatemalan companies) provide Express bus service between Flores and Belize City. In Belize City, buses arrive at and depart from the Marine Terminal on North Front Street near the Swing Bridge, and are not scheduled to stop anywhere else in Belize. Buses operate on Sundays and public holidays. Not all buses are air-conditioned. The agent in Belize City for these bus lines is Mundo Maya Travel located at the Brown Sugar Market Place on North Front Street, Belize City. Phone: (501) 223 1200 / 663 5957.
Email: mundomayatravels@yahoo.com

Linea Dorada: http://lineadorada.info/

Flores to Belize City (235 km / 146 miles / about 4 to 5 hours)
Fuente del Norte departs daily at 5:00 am; US$25.00
San Juan Bus departs daily at 5:00 am; US$ 25.00
Linea Dorada departs daily at 7:00 am*; US$ 25.00
* Departure of the Linea Dorada bus is sometimes delayed because it waits for a connecting bus from Guatemala City.

Belize City to Flores (235 km / 146 miles / about 4 to 5 hours)
Fuente del Norte departs daily at 9:30 am; US$25.00
Linea Dorada departs daily at 1:00 pm; US$ 25.00
San Juan Bus departs daily at 2:30 pm; US$ 25.00

This is the schedule from the agent, but there is another San Juan bus that departs from Chetumal at 7 am, goes to Belize City then to Guatemala.

(updated: 24 November 2011)