The pilot of the amateur-built Kitfox IV explained that after a thorough before-takeoff check of systems and instruments, the airplane performed as expected during the takeoff roll and initial climb. About 100 feet above ground level, the engine began "sputtering" and "losing power." The airplane lacked the altitude to return to the runway, so the pilot chose to land straight ahead in a swamp. Post accident examination of the engine revealed that both spark plugs in the number 3 cylinder did not spark. After exchanging the spark plugs, the engine started immediately and ran continuously without interruption.