The two dorsal fins were recognizable, even from the shoreline of Robert Moses State Park. They belonged to a lone basking shark, which seemed to be doing laps only about 25 yards out yesterday, prompting park officials to take the rare step of temporarily closing the beach for swimming.

All bathers were ordered out of the water at around 12:30 p.m. It didn't take that long to execute the directive, given that the water temperature was 63 degrees, and not many people had ventured in.

Suffolk Police flew a helicopter over the area, near parking field 5, and the U.S. Coast Guard sent a boat. An hour later, officials learned it was a six-foot-long basking shark, which is generally harmless to people, and reopened the waters.

"Once the Coast Guard said it's the type of shark that eats plankton, it wasn't a concern," said George Gorman, director of operations for Long Island State Parks. "Per its name, it seemed to be basking in the sun."

For the many families celebrating Father's Day and the last day of spring, the shark's cameo yesterday did not dissuade them from enjoying the 72-degree afternoon. Kids still played in the tide, and some buried siblings in the sand. Some families farther up the beach had no idea that the shark was even there, just before the first full day of summer.

"We saw a lot of people not in the water, but I just figured it was cold," said Paul Rock, 40, of Lindenhurst, who was there with wife Teresa and their six children.

The basking shark sightings began on June 10, when six were spotted swimming off Georgica Pond in the Hamptons.

The last time a part of Robert Moses was closed was last summer, after a shark report, but that sighting was never confirmed. Gorman could not recall another time when a shark was confirmed to be in the bathing area of one of the beaches.

At Robert Moses yesterday, a crowd swelled along the water's edge, especially when something ominous yanked on Louis Rea's fishing line. Then it quickly spooled out about 200 yards. "It had to be something like a shark," said Rea, 47, of Carmel, N.Y. "He did this last pull and then the line snapped. Otherwise I would have been in the water." In fact, the half-hour aquatic tug-of-water, during which the shark was visible, was one of the strongest fights the fisherman had ever felt.

The shark sighting wasn't scary for Kaitlin Shore, 9, and her sister Kristen, 7, who were at the beach to celebrate Father's Day with their dad Tim and family friend George Ghossn. "I was excited because I had never seen a shark before," said Kaitlin, of Huntington. "I wanted to swim with it." And to that request, her father gently replied no.