Missouri inmate Timothy Guerra appeals the district court’s preservice dismissal of his 42 U.S.C. § 1983 complaint against defendant prison personnel for failure to exhaust administrative remedies. We reverse.

An inmate’s failure to exhaust available administrative remedies, as required by 42 U.S.C. § 1997e(a), is an affirmative defense, which need not be pleaded by plaintiff, and which is subject to waiver by the defendant. See Nerness v. Johnson, 401 F.3d 874, 876 (8th Cir. 2005) (per curiam); Foulk v. Charrier, 262 F.3d 687, 697 (8th Cir. 2001). We disagree with the district court’s conclusion that it was evident from the face of Guerra’s complaint that he had *113not met section 1997e(a)’s requirement, and we conclude that Guerra’s complaint should not have been dismissed sua sponte on this basis. We also conclude that Guerra’s complaint, while lengthy and involving numerous claims, satisfied the requirements of Federal Rule of Civil Procedure 8(a).

Accordingly, we reverse the dismissal order, and remand to the district court for further proceedings.