BOZEMAN, Mont. (AP) _ A relatively mild earthquake centered south of Bozeman, Mont., was felt as far south as Yellowstone National Park, geologists said today.

The temblor, which measured 4.5 on the Richter scale, had its epicenter near Bear Trap Canyon, said Steve Custer, head of the Earth Sciences Department at Montana State University in Bozeman.

Custer was in bed when the earthquake occurred at 11:23 p.m. Tuesday.

''It was funny because I looked over at my wife and she thought I was kicking her and I thought she was kicking me,'' he said.

Two campers in Yellowstone National Park, which is about 80 miles southeast of Bear Trap Canyon, reported feeling the quake, according to park officials.

Geologists at the National Earthquake Information Center in Golden, Colo., said the earthquake was one of a series that has been shaking the area for about a week.

''This appeared to be the largest one in the series so far,'' said Wavery Person.

The Richter scale is a gauge of the energy released by an earthquake as measured by the ground motion recorded on a seismograph. A quake of magnitude 2.0 is the smallest normally felt by humans. A magnitude 3.5 quake can cause slight damage, while one of 5.0 can cause considerable damage.