The Census Bureau will include overseas military and civilian Defense Department personnel and their dependents in the 1990 census counts used for House reapportionment, a spokesman said yesterday. Inclusion of overseas military in the official state population totals that determine how many seats a state gets in the House occurred only once before, in 1970. The bureau's problem comes in deciding which states the individuals should be credited to. Aides said House census subcommittee Chairman Thomas C. Sawyer (D-Ohio) is pleased with the bureau decision because the overseas military and civilian defense employees clearly have the United States as their usual place of residence. Some experts fear that such a count would unduly benefit a handful of states that have no state income tax because many overseas service families favor such states for their official place of residence. Over the past year or so, many members of Congress have urged a count of the overseas military on grounds that exclusion of Americans serving their country abroad is unfair and irrational. Rep. Thomas J. Ridge (R-Pa.) and some others have also argued that not counting overseas personnel is objectionable in view of the bureau's intent to count illegal aliens living in the United States. The bureau's decision was described in testimony by acting director C. Louis Kincannon prepared for a Tuesday hearing by the subcommittee. The testimony was not delivered because the hearing was cut short when Ridge offered an amendment on the House floor to exclude illegal aliens from the 1990 count for reapportionment purposes. It was ruled out of order as legislation on an approprations bill. Kincannon's prepared testimony said, "The bureau will include overseas members of the armed forces, civilian Department of Defense employees, and their relatives in the 1990 population counts used for apportionment." Other civilian employees of the United States living abroad and other U.S. residents living abroad will not be included in the counts because locating and counting them is too difficult. Although Kincannon did not say so in his prepared testimony, sources said the bureau is expected to assign overseas military and civilian defense employees and their dependents to the last state where they lived for at least six months before going abroad.