Mats Hallin's first NHL hat trick led the New York Islanders to a 9-2 victory over the Flyers in Uniondale, N.Y., last night. The defending Stanley Cup champions hadn't beaten the Patrick Division leaders this season.

The Islanders are two points ahead of Washington in the battle for second place in the division, which Philadelphia leads by 10 points.

Hallin, who was celebrating his 25th birthday, scored once in the opening period as the Islanders took a 4-0 lead against Bob Froese, who shut them out in two previous games.

After defenseman Ken Morrow beat Froese with a 20-foot wrist shot at 4:58, Hallin converted Billy Carroll's pass to make it 2-0. Mike Bossy tipped in Dave Langevin's shot for his 54th goal at 10:54 and Butch Goring weaved through the Flyers defense and backhanded a shot over Froese's glove to make it 4-0.

The Flyers, who held a 3-0-2 edge over the Islanders this season, skated with more vigor in the second period, unleashing 20 shots at Islanders goalie Roland Melanson. Mark Howe scored on a backhander just eight seconds into the period ot make it 4-1.

After Hallin ripped in a wide-angle 30-footer at 6:26, Ilkka Sinisalo scored for Philadelphia. But Hallin knocked in a rebound of a Denis Potvin shot for his seventh goal of the season.

In the third period, Duane Sutter's shot trickled through Froese's pads at 3:57 to make it 7-2. Clark Gillies connected on a power play at 12:33 and Anders Kallur concluded the scoring at 18:29.