The title of world’s longest sperm actually belongs to a tiny fruit fly called Drosophila bifurca. When the coiled sperm is straightened out, it measures about 2 inches which is over 1,000 times longer than a human sperm. In fact, the testes of a fruit fly makes up 11 percent of the body mass of the male!

Turns out the very long sperm is evolutionarily driven by the just-as-long female reproductive tract, which is like an obstacle course, complete with harsh chemicals to weed out weak sperms. (Source)