BENSON, Ariz. (AP) _ A transcontinental Greyhound bus collided with an equipment-laden flatbed truck on Interstate 10, injuring more than 40 people, at least 10 of them seriously, state police said today.

No fatalities were reported, Department of Public Safety dispatcher Jerry Stiles said. The injured included a woman five or six months pregnant and her four children, and the bus driver, who lost at least one leg, officials said.

Stiles said the bus struck the rear of an 18-wheel semi-trailer flatbed that reportedly was carrying heavy iron castings.

The accident occurred Sunday night on a steep incline about four miles west of Benson, which is 45 miles southeast of Tucson.

Officials said there apparently were at least 43 ticketed passengers aboard the bus, plus the driver, but it was unclear whether the figure included several small children. All aboard received at least minor injuries.

Dr. Michael Gray, chief of staff at Benson Hospital, said officials told him that the first several feet of the front of the bus ''just accordioned backwards.''

Two helicopters shuttled injured passengers from Benson Hospital or directly from the crash site to trauma centers at two Tucson hospitals. Ambulances carried additional passengers to the hospitals.

Gray said 28 people were taken to the Benson emergency room. Sixteen of them were transferred to Tucson.

Tucson Medical Center spokesman Tom Reavis said 19 people arrived at the hospital. Of that number, 12 remained hospitalized and two were in critical condition, including a 2 1/2 -year-old boy who underwent surgery for a depressed skull fracture.

Reavis said at least five others were in guarded condition, including the bus driver, as Jerry Henson, 49, of Phoenix, who lost one leg.

The others listed as guarded at Tucson Medical Center included a 1-year-old girl, an 80-year-old woman and the truck owner, James Middleton, 58, of Houston.

Jane Baldwin, 49, of Fresno, Calif., was listed in poor condition; and Margo Romero, 21, of Houston, believed to be five to six months pregnant, was listed in fair to poor condition, though her fetus was believed to be stable.

Two other of her small children, J.J. and Daniel, were listed in satisfactory condition, Reavis said.

He said another seven passengers, including a family of six, were treated and released.

Two of 12 passengers transported to University Medical Center were admitted to the intensive care unit with head and internal injuries, with another seven admitted for observation and listed in guarded and stable condition, said spokeswoman Julietta Gonzalez.

George Gravley, Greyhound Lines Inc. public relations director in Dallas, said the bus was on a transcontinental run from New York to Los Angeles and had left El Paso, Texas, at 7:15 p.m. The accident occurred at 11:25 p.m. MST.

He said the company had an investigative team at the scene and acknowledged that the bus apparently had collided with the rear of the truck.

Stiles said the bus driver ''left about 250 feet of skid mark. He ran into the 18-wheeler. We don't know if there was someone in the lane to the left.''

''Why he ran into the 18-wheeler, why he didn't go over into the other lane we don't know,'' Stiles added. ''We probably won't know until we talk to him.''