In flies heterozygous for βTub85Dⁿ with a single copy of P{βTub85D-βTub56D.int}, the axoneme morphology in the testis is wild-type. However, 2 copies of P{βTub85D-βTub56D.int} in the heterozygous background (a 2:1 βTub56D:βTub85D ratio) produces sterile males. Meiosis and cytoplasmic microtubules are normal in these males, but spermatids have a mixture of normal axonemes, axonemes lacking the central pair, and axonemes with 10 doublets. In all cases, the basal bodies have the normal 9+2 morphology, and this arrangement maintained for at least 40μm; 10-doublet axonemes are present only in more distal regions of the sperm tail. The tenth doublet is inserted into axonemes de novo laterally and appears abruptly, without obvious precursor structures. Rarely, the loss of the 10th doublet is seen. Occasionally mature 10-doublet axonemes retain the central pair. Occasional axonemes containing more than 10 doublets are seen, but always in an open configuration. Doublet microtubules are also seen in the cytoplasm in these flies. When βTub56D^βTub85D.int is added to males homozygous for βTub85Dⁿ, abnormal spermatid axonemes are seen in the testis of these sterile males. At an early stage of axoneme development, instead of the normal 9+2 doublet architecture a 9+0 arrangement is seen. Axoneme morphogenesis is otherwise equivalent to wild-type at this stage. Most A-tubules have acquired the inner dynein arm, accessory microtubules have initiated as protofilament projections from each B-tubule. Some spoke linkers are present. The integrity of 9+0 axonemes is maintained for only a fraction of the normal 1.9mm sperm tail length (average 20μm) Distally axonemes lose coherent organisation or terminate.