Cilia and flagella play multiple essential roles in animal development and cell 
physiology. Defective cilium assembly or motility represents the etiological 
basis for a growing number of human diseases. Therefore, how cilia and flagella 
assemble and the processes that drive motility are essential for understanding 
these diseases. Here we show that Drosophila Bld10, the ortholog of 
Chlamydomonas reinhardtii Bld10p and human Cep135, is a ubiquitous centriolar 
protein that also localizes to the spermatid basal body. Mutants that lack Bld10 
assemble centrioles and form functional centrosomes, but centrioles and 
spermatid basal bodies are short in length. bld10 mutant flies are viable but 
male sterile, producing immotile sperm whose axonemes are deficient in the 
central pair of microtubules. These results show that Drosophila Bld10 is 
required for centriole and axoneme assembly to confer cilium motility.