Columbia Federal Savings and Loan Association, the District's third-largest thrift institution, is seeking to merge with First Federal Savings and Loan Association of Washington, the sixth-largest. A formal application is expected to be filed in about two weeks.

If the merger goes through, Columbia -- with $543.3 million in assets -- and First -- with $387.5 million -- would become the second-largest S&L in the city, displacing National Permanent, which has assets of $708.1 million. National Permanent also is rumored to be looking for a merger partner.

The No. 1 thrift remains Perpetual (assets of $1.037 billion), which announced last May it intended to merge with American, the fifth-biggest. Earlier in the spring, Capital City and Northwestern, which ranked 23rd and 34th, respectively, in size, announced a merger. Also Interstate, fourth-largest, and Metropolis, 33rd, decided to join forces.

Columbia itself is the result of mergers since 1957 with Mutual, District Building & Loan, and Hamilton Federal. Its president, T. William Blumenauer, recently predicted more mergers would occur as savings and loans seek to grow to meet competition from commercial banks. Like all mergers, the Columbia-First coupling is subject to approval by the Justice Department's antitrust division.

Blumenauer, who has been with Columbia since 1938, is expected to serve as president of the merged institutions until his retirement in a few years. His heir apparent is Dewitt T. Hartwell, president of First Federal.

Columbia has eight offices in Northwest Washington and one in Southeast. First Federal has five offices, including one in Southwest and one in Bethesda.