About 80 buses were set to leave Galveston yesterday bound for shelters 160km north in Huntsville.
The buses were part of a mandatory evacuation that was ordered by officials in Galveston County, which has a population of nearly 267,000.
"We've always asked people to leave earlier, but because of Katrina, they are now listening to us and they're leaving as we say," Galveston Mayor Lyda Ann Thomas said.
The Pentagon stationed coordinating officers and staff at Tallahassee, Florida, and Austin, Texas, to assist storm preparations and recovery.
The USS Bataan, an amphibious assault ship, was off Florida's Atlantic coast near Jacksonville, preparing to follow behind Rita to support relief efforts.