The internal structures of the tail of male Caenorhabditis elegans nematodes are made by the descendants of four cells (B, Y, F, and U) which divide only in males. These cells are also present in hermaphrodites, where they have minor structural roles in the rectum. Here we show that the gene mab-9 is required for the correct development of two of these male-specific blast cells, B and F. In mutant males, the lineages of B and F resemble those of Y and U, respectively. These abnormal lineages lead to grossly defective male tails. We suggest that in mab-9 males the identities of B and F are transformed into Y and U, their respective anterior neighbors. The case for the F-to-U transformation is less strong than for the B-to-Y transformation because the wild-type lineages of F and U are very similar. Some mab-9 hermaphrodites are constipated as a result of abnormal rectal structure. This may be the result of an analogous fate transformation. mab-9 worms of both sexes are slightly uncoordinated. We propose that the fates of the four rectal cells are initially specified as two pairs (B and Y, F and U) and that the function of mab-9 in both sexes is to differentiate the posterior member of each pair from its anterior neighbor.