Miss. Code Ann. § 25-11-409: Miss. Code Ann. § 25-11-409

Eligible employees initially employed on or after July 1, 1990, shall elect to participate in the optional retirement program within thirty (30) days after (i) entry into state service, or (ii) the effective date of the optional retirement program, whichever is later. The election must be made in writing and filed with the board of trustees and will be effective as of the date of employment. If an eligible employee fails to timely make the election provided in this section, he shall become a member of the Public Employees' Retirement System of Mississippi in accordance with Article 3 of this chapter.