MENTALLY RETARDED citizens won a big victory in the Supreme Court last week in a case that should encourage the development of community residence programs and raise public understanding of the needs and qualities of these special citizens. At immediate issue was the question of whether the city council of Cleburne, Tex., could use its zoning powers to block the operation of a group home for the retarded in that community. All nine justices found that requiring a special permit for this group -- but not, for example, for a boarding house, a hospital or a fraternity house -- is an unconstitutional violation of the equal protection rights of the retarded.

The justices split 6-3, however, on the standard that courts should apply in reviewing statutes affecting these handicapped people. The equal protection clause of the 14th Amendment requires that all persons similarly situated should be treated alike. But states may enact laws under which some classes of people are treated differently if the classification is rationally related to a state interest.

Some kinds of classifications are inherently suspect. When a statute differentiates on the grounds of sex, for example, it will only be upheld if the classification is substantially related to a sufficiently important government interest. Distinctions on grounds of race or national origin must meet an even higher test and will only be sustained if they are necessary because of a compelling state interest.

Three justices believe that legislation treating the mentally retarded differently from others should be considered inherently suspect and subject to the highest level of scrutiny by the courts. But the majority held that such classification is often reasonable. The mentally retarded, in fact, are significantly different from other citizens, and society must often take these differences into consideration. Special education programs and different rules about military service, driver's licenses and civil service jobs, for example, are all legitimate. Legislative decisions in this area need not be unduly burdened by special judicial scrutiny.

The Supreme Court's decision should please those who work on behalf of the retarded, even though a comprehensive, strict new rule for judging statutes affecting them was not adopted. Justice Byron White's opinion detailed and praised the growing number of government programs directed at helping these citizens; it reflected optimism about continued public interest and concern. On the important question of Cleburne's group homes for the retarded, the opinion is clear. The statute's discrimination against this group rests on irrational prejudice and will not be tolerated. The ruling should provide important guidance to thousands of communities across the country.