CG3875 is a young duplicate gene of kep1 family originated recently in Drosophila melanogaster (D. melanogaster) species complex (including D. melanogaster, D. simulans, D. mauritiana and D. sechellia) after it split from D. yakuba. It encodes an RNA-binding protein containing a single maxi-KH domain. Characterization of loss-of-function phenotypes of CG3875 mutants generated by gene targeting demonstrated that CG3875 null males display a seriously reduced fertility compared with wildtype males and most of the null males are completely sterile. Further cytological identification of CG3875 null males suggested that CG3875 plays an important role in spermiogenesis processes including sperm individualization and sperm coiling. In addition, CG3875 is also essential for the formation of outer dynein arm of sperm axoneme. In order to identify the molecular mechanism responsible for the involvement of CG3875 in spermiogenesis and structural integrity of sperm axoneme, we performed microarray analysis to identify transcripts whose levels are altered in the testes of CG3875 null males.