BBVA Compass Bancshares Inc. mismanaged its 401(k) plan by keeping more than $100 million in plan assets in a fund that returned no more than .01%, according to a new lawsuit.

The proposed class action, filed July 18 by two BBVA 401(k) plan participants, challenges the decision to keep nearly one-fifth of the plan’s $931 million in assets in the Vanguard Money Market Fund, which participants say was unsuitable as an income-producing retirement investment. The BBVA defendants are accused of effectively “stuffing money into a mattress,” causing plan losses of nearly $8 million and violating their fiduciary duties under the...