SANTA MONICA, Calif. (AP) _ A man who spent 18 days in Soviet custody after walking across the frozen Bering Strait from Alaska says he also wasn't in the United States legally but now wants to become a citizen.

Lazaro Ruiz Castro, 27, said he wasn't mistreated by his captors but was disappointed he couldn't see more of Soviet society, according to a report published Friday.

Castro spoke by telephone from Nome, Alaska, to the Santa Monica Evening Outlook.

''It was an exciting adventure,'' Castro said. ''But my objective was to find out about Soviet culture, and I didn't see much. I was in prison all of the time. I wouldn't recommend it for anyone.''

The wanderer walked across three miles of ice from the U.S.-owned Little Diomede Island to Big Diomede on the Soviet side Jan. 15. He was returned by the Soviets on Monday.

A native of Mexico, Castro said he entered the United States illegally in 1980 and has lived near Los Angeles. U.S. officials say he may qualify for amnesty, and eventually citizenship, under the new immigration law.

Castro said he was visiting Alaska when he decided to walk to the Soviet Union. He was taken prisoner by five soldiers and held in a small cabin for 10 days, Castro said. He was later taken inland to a small village and kept in an apartment house.

Soviet officials questioned him extensively, Castro said. ''They thought I was a spy.''

Castro said U.S. authorities didn't trust him either. The FBI wanted to know if he had brought back any Soviet propaganda.