The fine structure of mature spermatozoon of the strepsipteranHalictophagus chilensisHofmann was studied by light and transmission electron microscopy. The spermatozoon was seen to have an elongated head and a tail with a ≪9 + 9 + 2≫ axoneme and two mitochondrial derivatives. The mono‐layered acrosome is conical in shape and the long nucleus has an internal channel of uncondensed chromatin. The tail is five times longer than the head and the end of the axoneme shows progressive loss of microtubules. These findings are compared with the sperm ultrastructure of other strepsipteran species studied so far.