This document contains a final regulation revising the definition of when certain monies which a participant pays to, or has withheld by, an employer for contribution to an employee benefit plan are ``plan assets'' for purposes of Title I of the Employee Retirement Income Security Act of 1974 (ERISA) and the related prohibited transaction provisions of the Internal Revenue Code (the Code). The final regulation provides that participant contributions to employee pension benefit plans become plan assets on the earliest date that they can reasonably be segregated from the employer's general assets, but in no event later than the 15th business day of the month following the month in which the participant contributions are withheld or received by the employer. The final regulation establishes a procedure by which an employer that sponsors a pension plan may obtain an extension of this maximum period for an additional 10 business days with respect to participant contributions received or withheld in a single month. With respect to employee welfare benefit plans only, the final regulation leaves unchanged the current regulation, which provides that participant contributions become plan assets as of the earliest date on which they can reasonably be segregated but in no event later than 90 days from the date on which the participant contributions were received or withheld by the employer. This rule provides guidance to employers that sponsor contributory pension and welfare plans, including plans complying with section 401(k) of the Code, as well as fiduciaries, participants, and beneficiaries of such plans.