An 18-year-old day laborer from Guatemala was charged yesterday with beating his girlfriend's 3-month-old daughter to death in Vineland, N.J.

NJ Transit police arrested Randy Lopez-Rivas in Camden as he was walking near the Walter Rand Transportation Center, police said. He was charged with murder. When the 21-year-old mother, Guillermina Matias-Lopez, arrived home from work Tuesday, Lopez-Rivas and the baby were gone, authorities said. Police found the girl, Kimberly, in a garbage bag behind a clothes washer in the basement of the Vineland house where the couple rented an apartment.

Matias-Lopez had left Kimberly with Lopez-Rivas, who was not the father, before going to work at 4:45 a.m., Cumberland County Prosecutor Ronald Casella said. Lopez-Rivas turned up alone at the nearby home of a woman who often took care of the infant and asked for a ride to the bus station, where he got on a Philadelphia-bound bus, Casella said.

When Matias-Lopez arrived at the baby-sitter's house about 7:45 p.m., she was alarmed that her child was not there, Casella said. She could not find the infant at home, either. Kimberly was found at 11 p.m., after more than two hours of searching.

Authorities said they believed the baby had been beaten to death. An autopsy was being conducted yesterday.

Officials said they didn't know why Lopez-Rivas would have struck the baby.