The small marble tombstone and white wooden cross marking the grave of Robert F. Kennedy were stolen this weekend from Arlington National Cemetery, U.S. Park Police reported yesterday.

Thieves apparently scaled a stone wall and a six-foot fence surrounding the 600-acre cemetery, then dug out the one-by-two-foot Vermont marble marker and uprooted the two-foot white wooden cross, cemetery officials said.

"It was a terrible thing," said Raymond J. Costanzo, the park superintendent. "The Kennedys have had enough grief without having some prankster add to it."

The theft occurred between 6 p.m. Saturday, when the iron cemetery gates were locked for the evening, and 8:30 a.m. Sunday, when Arlington custodians making customary morning clean-up rounds discovered that the cross and tombstone were missing, Costanzo said.

Costanzo, who lives on the cemetery grounds, has been superintendant for 10 years. He said yesterday marked the first theft of a grave marker that he could recall.

Park police, who are investigating the incident, said they have no suspects and no idea what the motive for the theft may have been.

Erected four years ago, the Robert Kennedy grave markers were modest ones at the request of his family, Costanzo said. Materials were procured through the Veterans Administration at a cost of about $150, he said. The white marble was inscribed simply "Robert Francis Kennedy/1925-1968."

The late senator's grave is on a hillside about 25 yards from the resting place of his brother, President John F. Kennedy. The slain president's gravesite was not disturbed, Costanzo said.

The cemetery is staffed by Army personnel by day, and U.S. park police make occasional patrols of the grounds at night after the gates are locked, according to Costanzo. He said he plans discussions with Army officials about whether tighter nighttime security is needed.

Costanzo also said he would ask the VA today to make an "expedited" order for a new tombstone.