A former U.S. ambassador to Costa Rica pleaded guilty yesterday in U.S. District Court in Alexandria to helping an illegal alien enter the U.S. to work as the family domestic and to paying the woman less than the required minimum wage.

Raymond L. Telles, 61, who served an ambassador from 1961 to 1967 and on the Equal Employment Opportunity Commission from 1971 to last year, originally was charged with conspiring with his wife to violate both immigration and labor laws by bringing a Cost Rican woman into the U.S. under false pretenses. Telles and his wife also were charged with concealing information from Immigration and Naturalization Service investigators.

Yesterday, Telles told U.S. District Court Judge Robert R. Merhige that he wanted to plead quilty to aiding and abetting an illegal alien to enter the country and violating the minimum wage law - two charges that carry misemeanor penalties - rather than be tried on the original, felony grand jury charges. Telles made his guilty pleas after one prosecution witness testified at his trial. Prosecutors accepted the pleas.

As a result of Telles guilty pleas, U.S. attorneys agreed to drop charges against Telles wife, Dalphine, 59, who was admitted Sunday to the psychistric ward of the Alexandria Hospital, according to Brian Gettings, Telles' attorney. Gettings said his client decided to plead guilty largely because of his wife's condition.

Dr. Franklin J. Pepper, an Alexandria psychiatrist, told Merhige bfore the trial began that Mrs. Telles was suffering from depression and that another psychiatrist on the hospital's staff had found that she showed "suicidal tendencies." Pepper added that he found her fit to stand trial and serve as a defense witness yesterday.

Telles faces six months in jail, a $500 fine or both for violating the immigration law and a fine of up to $10,000 plus damages for failing to pay proper wages to the Costa Rican woman, according to Assistant U.S. Attorney John F. Kane. The damages, according to U.S. law, will amount to double sum of money Telles owes the woman in back pay and could total $2,000, Gettings said.

Had he stood trial and been convicted on the original charges, Telles could have been sent to prison for as long as 15 years and been fined a total $15,000 according to U.S. law.

Telles admitted to helping Marielos Alverez-Bran enter the U.S. in October, 1974, under nonimmigrant-visitor status with the intention of hiring her as a maid. Under immigration laws, the woman would have required Department of Labor certification and an immigrant's visa to work and resident permanently in the U.S.

He also pleaded quilty to paying Alverez-Bran - who worked for his family until May, 1975 - $12.50 a week for a 60-hour work week during the first four months in this country, then raising her wages to $25 a week, while providing her room and board. According to Kane, the woman was entitled at that time to receive $2.25 an hour, or $135 a week.

Kane said Alverez-Bran Telles has been permitted to remain in the U.S. because of the Telles investigation and that she has received special permission from the immgration service to support herself as a domestic.

A sentencing date has not been set for Telles, who was twice mayor of El Paso, Tex., the town where he was born and raised. He lives at 6808 Whittier Blvd., Bethesda.