As part of a survey of the metazoan parasites of elasmobranchs of the Gulf of California, Mexico, the spiral intestines of 10 pelagic thresher sharks Alopias pelagicus and one bigeye thresher shark A. superciliosus were examined for tapeworms. Eight of the A. pelagicus specimens examined were found to host Litobothrium amplifica and L. daileyi. Both tapeworm species are redescribed based on examination of this new material with light and scanning electron microscopy, and the ranges of most of the measurements for these species are expanded; scanning electron micrographs and detailed illustrations and measurements of their segment anatomy are presented for the first time. An argument is made that the identification of the original host specimens of these species was in error and that A. pelagicus is likely to be the correct original host. In addition, L. nickoli n. sp., a third species in the genus hosted by A. pelagicus, was found in three of the 10 individual hosts examined. This species differs from all six known Litobothrium species in the form of the pseudosegments of the scolex, the anterior two being essentially non-cruciform, while the latter three are distinctly cruciform. All other species possess either no non-cruciform or at most one non-cruciform segment anteriorly. The single specimen of A. superciliosus examined was found to host the new species, L. janovyi. This species differs from L. coniformis, L. gracile and L. amsichensis in its possession of four rather than three, three and five cruciform pseudosegments, respectively. It lacks the modificiations of the fourth pseudosegment seen in L. amplifica and lacks the anterior non-cruciform fifth pseudosegment found in L. daileyi. It most closely resembles L. alopias but differs among other features in its greater total length, greater number of segments and longer mature segments. SEM of the four species collected from the Gulf of California as well as material of L. amsichensis from the goblin shark Mitsukurina owstoni that hosted the type-specimens of this species show that all surfaces of the body of all five species bear a dense covering of long filiform microtriches. L. amplifica bears a single row of large spine-like structures throughout most of the posterior margins of the first and second cruciform pseudosegment only. L. daileyi possesses one to two rows of overlapping spine-like structures on the posterior margins of the first four pseudosegments with the exception of the medial projections. The fifth pseudosegment lacks these structures. L. janovyi n. sp. bears spine-like structures on the lateral margins of only the third and fourth pseudosegments. L. nickoli n. sp. bears spine-like structures throughout the posterior margins of the first and second pseudosegments, and throughout the posterior margins of the third and fourth pseudosegments with the exception of the medial projections; the fifth pseudosegment lacks these structures. L. amsichensis bears no structures that could be considered to be spine-like on any of its pseuosegments, but possesses a border of densely arranged larger microtriches on the posterior margin of all five pseudosegments. A key to the species is included.