The Guam Department of Agriculture said hundreds of large spiders and thousands of smaller ones were seen when stevedores began offloading insulation and beams for housing units from the ship, the M.V. Altavia.
The cargo was returned to the ship, and the Agriculture Department on Friday ordered that the ship not be allowed to dock. It was last ported in South Korea.
"When you get this many from this many various sizes, it's definitely an infestation," said Department of Agriculture Director Joseph Torres.
Agriculture officials said they didn't know what type of spiders were on the ship. But they said it's a type that is not normally found on Guam and there was concern the spiders could damage the island's environment.
"It's not on Guam," Torres said. "We don't want it here."
The ship was carrying housing units and accessories that were to be used at a work force village expected to house up to 18,000 temporary workers.
Before the ship initially docked, officers with the Guam Customs and Quarantine Agency boarded the ship and gave clearance for cargo to be unlocked for offloading, said Bernadette Meno, an administrator for the Port Authority of Guam.
But when port workers saw the spiders, the containers were ordered back on the ship and it was anchored in the harbor. The decision was later made not to let the ship return.
Marianas Steamship Agencies Inc. arranged for the ship's arrival and departure. Its vice president, Richard P. Sablan, said he will follow orders of customs, agriculture and U.S. Coast Guard officials.