MATAGALPA, Nicaragua (AP) _ Benjamin Linder once dressed in a clown suit, painted his face with red dots and rode through the village on a unicycle shouting ''Death to Measles 3/8'' to get children to follow him to the vaccination post, a friend recalled.

Other friends and relatives of Linder, the 27-year-old Oregon engineer and pro-Sandinista volunteer killed Tuesday by Contra rebels, described him as a cheeful idealist.

''He was a great guy who did his best to make the world a better place for people to live, and because of that he got killed,'' his father, David, said Wednesday outside the family's Portland home.

Linder went to Nicaragua in 1983 shortly after graduating from the University of Washington with an engineering degree. He first worked as a circus clown, entertaining children with his expert juggling and unicycle antics.

Even after he put his education to work in the construction of small power plants in remote regions, he continued entertaining children.

Mira Brown, a fellow volunteer from Boston, said Linder ''was always very happy.''

She worked with him in building a hydroelectric plant that brought electricity for the first time to El Cua, a cluster of adobe houses near Matagalpa.

The slight, red-bearded Linder worked for the Nicaraguan Appropriate Technology Project, known as NICAT and based in Bellingham, Wash.

Alejandro Morales, a Nicaraguan colleague, said the young American was a ''highly qualified technician'' and a fine clown.

''Sometimes when villages would have parties, he would dress up as a clown,'' Morales said. ''He would fascinate the children, including my 7-year- old daughter, with juggling acts, some on a unicycle he owned, and other tricks. He was special in his own way.''

Ms. Brown recalled the campaign to vaccinate children in El Cua.

She said Linder put on his clown suit, painted dots on his face and went around on his unicycle shouting ''Death to Measles 3/8'' to entice the children to the vaccination center.

Others said he accepted hardship cheerfully and acted without fear.

''It embarrasses me to say this, but he went to work where Nicaraguans were afraid to go,'' Morales said.

Dr. Anne Lifflander, an American who worked for the Ministry of Health in Nicaragua from 1983 to 1985, said she arrived in the country at about the same time as Linder.

''At that time, we were both eager to use our skills to help the Nicaraguan people,'' she said. ''Ben was certainly committed to work. He also was committed to life, and he wanted to learn about the Nicaraguans' lives.''

Lifflander said she and Linder had many discussions about whether ''to stay or to leave and how much danger we should face.

''It was worth it to him,'' she said. ''He wanted to bring electricity to the poor people of Nicaragua.''