Can a frivolous complaint be dismissed pursuant to Rule 12(b)(6)?

1 Defendants contend that the district court dismissed the complaint as frivolous pursuant to 28 U.S.C. 1915(d). Under section 1915(d), a frivolous complaint may be dismissed sua sponte prior to service of process. Nietzke v. Williams, 490 U.S. 319, 324 (1989). Here, the record reveals that the defendants were served and filed a motion to dismiss prior to the district court's dismissal. Accordingly, we construe the district court's dismissal as pursuant to Rule 12(b)(6).