The transmission ratio distortion seen in males heterozygous for a mouse t-complex has been explained on the basis of trans-acting distorter genes, having a harmful effect on a responder gene. The t-complex form of the responder is relatively resistant to these harmful effects and hence is preferentially transmitted. Animals homozygous for the t-complex responder would be expected to show equal transmission of the two homologous chromosomes, but this is not always so. Studies described in this paper have shown differences among complete t's in their transmission when opposite a constant responder carrying partial t-haplotype. In addition, the proximal partial haplotypes th49 and tw18, both derived from tw5 but of different lengths, behave differently when opposite a responder. The three central partial haplotypes, tlowH, tlow2H and tlow3H, also differ, in that tlow3H shows lower transmission than tlowH or tloW2H when opposite either wild-type, or another responder, or distorter genes. These results can be explained either on the basis of differences in the responder region of various haplotypes, including the possibility of varying numbers of copies of the relevant sequences, or on the basis of differences in cis-acting (as opposed to trans-acting) distorter genes.