The United States, which 10 years ago banned the sale of small pet turtles because they carry the potentially deadly salmonella bacteria, continues to export up to 4 million turtles a year to countries where there is no ban.

Those turtles could be causing as many as 400,000 cases of salmonellosis a year, the Journal of the American Medical Association reports.

The disease, which often affects children, causes fever, severe gastrointestinal problems and, in one case in 1,000, death.

"Undaunted and without conscience," the journal says in an editorial, "the industry continues to export its lethal product beyond the reach of legal constraints."

Turtle distributors, most of them based in Louisiana, say antibiotic treatment of turtle eggs can make the turtles salmonella-free, and packing them in bubbles can keep them salmonella-free upon delivery. But critics say this technique is not being used, and even if it were, the turtles could become contaminated later.

Use of antibiotics, says Dr. Robert V. Tauxe of the Centers for Disease Control, could lead to the proliferation of resistant strains of salmonella, and doctors are "wringing their hands" about that prospect.

In Puerto Rico, the focus of the journal study, 12 to 17 percent of the salmonella cases in infants could be traced to turtles. All turtles tested carried the bacteria, Tauxe's study showed.

Among the major turtle importers in 1983, according to the United States Fish and Wildlife Service, were Japan (1.2 million), France (266,000) and Italy (188,000).

"What are urgently needed," the journal's editorial says, "are export constraints . . . so that people of distant lands are protected against pet turtles and other knowingly dangerous products we export, which include but are not limited to pesticides, obsolete pharmaceuticals and toxic wastes."