A teenage day-laborer from Guatemala was charged yesterday with beating to death his girlfriend's 3-month-old daughter.

New Jersey Transit Police arrested Randy Lopez-Rivas, 18, as he was walking near the Walter Rand Transportation Center in Camden, police said. He was charged with murder.

Lopez-Rivas had disappeared from his Vineland apartment before his girlfriend, Guillermina Matias-Lopez, 21, came home from work and realized her baby was missing.

The baby was found later in a garbage bag behind a clothes washer in the basement of the house where the couple was renting an apartment.

Cumberland County Prosecutor Ronald Casella said Matias-Lopez had left home for work at 4:45 a.m. Tuesday.

She left the girl, Kimberly Lopez-Rivas, with Lopez-Rivas, who was not the baby's father.

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 hopped onto a Philadelphia-bound bus. Authorities wondered if he would try to leave the country, or perhaps head to Boston, where he has relatives.

Investigators were trying to figure out whether that raised any alarms with the sitter and whether she asked about the baby's whereabouts, Casella said.

When Matias-Lopez arrived at the 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.

At 11 p.m., after more than two hours of searching, police found the baby behind the washing machine. *