A measure granting minority status to Cajuns has been approved by the Louisiana House of Representatives over the objections of black lawmakers who say the measure threatens black economic development, Cajuns are descendants of French settlers expelled from Canada who settled in the steamy bayou country of south Louisiana in the 1750's. The Louisiana Almanac estimates the number of Cajuns in the state between 200,000 and 300,000. The state's population is about 4 million.

State Senator Jon Johnson said today that black lawmakers would fight the legislation because the counting of Cajuns as members of a minority group would dilute the black share of employment on state projects. Such projects must by law give 10 percent of its jobs to minority workers.

The bill is now in the state Senate, where Mr. Johnson, a member of the Legislative Black Caucus, said it must be killed.

''I for one have absolutely no intention at all of letting a group of individuals who have enjoyed anything from serving as governor of this state on down who are going to come in and make an attempt to share as a minority,'' said Mr. Johnson, a New Orleans Democrat.