More cargo is being moved by refrigerated or temperature-controlled containers. But the trend is causing problems for ocean carriers, who must find cargo to fill their ships for the opposite direction.

Refrigerated containers have become more popular as additional trade routes have opened with countries in South America, the South Pacific and Africa.When most of the cargo was moved by breakbulk ships - meaning most of the cargo was transported on pallets - trips to Australia or Chile would take too long and fruits and vegetables from those places would spoil in transit.

Carriers attacked the problem of long trip times with temperature- controlled ships. Then came containerization, which eventually led to refrigerated containers.

The shipping industry developed a more sophisticated line of containers designed to transport refrigerated cargo. Ocean carriers launched fleets of special ships to carry the containers.

The special service has helped trade flourish between the United States and places like Chile and Australia.

Chilean fruit and vegetables, for example, have boosted cargo numbers at ports along the East Coast like Philadelphia; Wilmington, Del.; and Wilmington, N.C.

Philadelphia also has become a major load center for Australian beef, which is carried in refrigerated containers.

But, while carriers are bringing the cooler cargo into U.S. ports, they must take what they can get for the outbound leg.

And, because the ships are built for refrigerated cargo, they cannot pick up just any cargo.

"In the old days, on breakbulk ships, they could load anything," said Jim Toy, the chief mate aboard the Sea Fox, a containership that carriers regular and refrigerated containers between South America and the U.S. East Coast for Crowley American Transport. "But for this ship right now, if it's not in a container, we can't load it."

But the ship also handles cargo that can be driven on or off the vessel, such as tractors.

"It limits you," Mr. Toy said.

Some lines, like Chilean Lines, which carries fruit and vegetables from Chile to Philadelphia, have started carrying cars on the backhaul trip to be sold in South America.

"This was something the Chileans developed themselves," said William McLaughlin, a spokesman for the Philadelphia Regional Port Authority.

"Some cars fit very easily into containers," Mr. Toy said. "We have some aboard now."

Other lines are looking for a similar mix. Cool Carriers carries secondhand cars from Europe to southern Africa and South America, and from Japan to South America. The line carries other dry cargoes, including fertilizer, paper products and timber on backhaul voyages.

Lauritzen Reefers of Denmark also is carrying more general cargo, including cars, on return trips.

An industry trend should help the ocean carriers: More and more cargo is being containerized.