1. If fish fillets are large, cut into 4 serving pieces; pat dry. Arrange fish, thickest parts to outside edges, in square dish, 8 x 8 x 2 inches. Squeeze lime over fish; sprinkle with cilantro and chili powder.
2. Cover tightly and microwave on high 5 to 7 minutes, rotating dish 1/2 turn after 3 minutes, until fish flakes easily with fork. Let stand covered 2 minutes. Serve with salsa and, if desired, lime wedges.