The Supreme Court ruled yesterday that the burglary indictment of a Mexican-American in Texas was faulty because there weren't enough Spanish-surnamed people on the grand jury.

The court, in a 5-to-4 decision, upheld the claim of Rodrigo Partida that there was "purposeful discrimination" in the makeup of the grand jury, even though Mexican-Americans constitute a "governing majority" of Hidalgo County, Tex.

Partida was indicted in March, 1972, for burglary of a private residence at night with intent to rape. He was convicted by a state jury and sentenced to eight years in prison.

His claim of discrimination stemmed from the fact that over a 10-year period the average percentage of Spanish surnames on grand juries was 39 per cent, in a county where 79 per cent of the population was Mexican-American. THe state will now have to decide whether to seek a new indictment of Partida.

The majority decision, written by Justice Harry A. Blackmun, said the state "failed to rebut the presumption of purposeful discrimination by competent testimony."

But Justice Lewis F. Powell Jr., dissenting to a ruling he termed "wholly uprecented," said he felt a sense of unreality that justices in Washington were using inferences from statistics "that the Mexican-Americans who control the levers of power in this remote border county are manipulating them to discriminate against themselves."