WASHINGTON (AP) _ A Phoenix holding company acquired 15 insolvent Texas thrifts Thursday for $120 million and $1.85 billion in assistance from the Federal Savings and Loan Insurance Corp., federal regulators annouced.

The transaction was part of the Southwest Plan launched last May to rid Texas of ailing savings and loans and brings to 69 the number of thrifts consolidated or acquired through the plan.

The 15 thrifts, located throughout the state, are being consolidated into a newly chartered federal stock association - Consolidated Federal Bank FSB - that will open on Friday, the Federal Home Loan Bank Board said.

The thrifts affected by Thursday's acquisition include Commodore Savings Association, Dallas; Mesquite Savings and Loan Association, Mesquite; Mineral Wells Savings and Loan Association, Mineral Wells; Sentry Savings, Slaton; Interwest Savings Association, Fort Worth; Metroplex Federal Savings Association, Hurst; North Park Savings Association, Richardson; and Southern Federal Banc SLA, Lancaster.

Other thrifts involved are First Western Savings and Loan Association, Colorado City; First Federal Savings and Loan Association of Big Spring, Big Spring; Hi-Plains Savings and Loan Association, Hereford; Reliance Savings Association, Houston; Vista Savings Association, Odessa; Home Savings, Lufkin; and Lamesa Federal Savings and Loan Association, Lamesa.

The holding company acquiring the institutions is headed by James M. Fail, chairman of several insurance companies, including Lifeshares Service Co., First United, Inc. and LSC Marketing, Inc.

The FSLIC is providing an $837 million, 10-year note to restore net worth, capital loss coverage on certain covered assets and a yield maintenance agreement on certain assets, the bank board said.

The FSLIC will receive 25 percent of the tax benefits realized on a consolidated basis and warrants for 20 percent of consolidated common stock.

''With this investment today, we have now attracted $530.8 million of capital to the Texas thrift industry, which demonstrates the interest generated among potential acquirers by our plan,'' said M. Danny Wall, bank board chairman.

''We still have a few days left in 1988 and the number of thrifts we'll act on by year-end, as well as elsewhere, will likely be even higher.''

Wall said more than half of the sick Texas thrifts have been consolidated and to date 175 thrifts have been resolved.