The Bureau of the Census, as Spencer Rich pointed out {Federal Page, Jan. 12}, faces a major problem in taking the decennial population census in April 1990 -- namely, whom to include in the census count.

One possible solution to this dilemma would be for Congress to amend the census statute to provide for the following two separate categories of persons found in the country on April 1, 1990 (excluding legal nonresidents such as foreign tourists and diplomatic personnel):

1. Persons to be used in the census for reapportionment of congressional representatives among the states would include all citizens, lawful permanent resident aliens and members of the armed forces and civilian employees of the Department of Defense and families assigned to a post outside the United States.

2. Persons to be used in the census for purposes other than reapportionment would include, in addition to Category 1, conditional and temporary resident aliens and other aliens.

To divide the census into the above two groups, the following check boxes could be included in the name section of the questionnaire: (a) native-born or naturalized citizen, (b) permanent resident alien, (c) conditional or temporary resident alien and (d) other alien. Also, if the mailing address is not a legal address for voting purposes, a box for the zip code of the legal address should be included.

Providing for a count of all legal permanent residents should fulfill the constitutional requirement for apportionment of representatives. Adding all other residents -- who may need education or other public services -- to the census count would provide a fair basis for the sharing of federal, state or local funds. REESE R. MORGAN Alexandria The writer was an employee of the Census Bureau for 25 years.