The microtubule cytoskeleton, important for cell division and motility, is regulated by a complex system of microtubule-associated proteins and motors. Microtubule inner proteins (MIPs) are a novel group of Microtubule associated proteins (MAPs) that are localized inside the microtubule lumen. Previously, known MIPS consisted of single proteins or small protein complexes. The tips of flagella possess a region containing only singlet microtubules. We have examined this singlet zone in intact human sperm tails by cryo-electron tomography followed by subvolume averaging and report the presence of a novel structure on the interior of the microtubules that we call: TAILS (Tail Axoneme Intra-Lumenal Spirals). This structure spans the entire singlet zone (several micrometers) and forms a left-handed interrupted helix with 8 nm rise and 12 nm pitch. TAILS is coaxial with the surrounding microtubule helix, which is consistent with identical subunits binding directly to the interior microtubule wall but leaves a gap over the microtubule seam. This is the first higher order structure found inside of a microtubule lumen. We suggest that TAILS may stabilize microtubules, enable rapid swimming, or play a role in controlling the direction in which spermatozoa swim.