Since the realization that the vesicular transport machinery is conserved from yeast to vertebrate neurons, much interest in the proteins involved has been generated. Here we describe a new type of mammalian YPT/Rab protein, named Rab11b, that is most abundantly expressed in brain, heart, and testis. It has all the hallmarks of a YPT3/Rab11 protein, but is more closely related to a ypt3-related gene isolated from fish (97% homology) than to the previously described four mammalian Rab11 genes (89% homology). The mammalian Rab11 genes discovered previously are, by contrast, 100% identical to each other. The genomic structure of a mammalian Rab11 protein is described for the first time. It is surprisingly asymmetrical, with 96% of the coding sequence contained in one-third of the transcription unit. Two copies of this gene are mapped to mouse chromosomes 1 and 17. The fine structural analysis of Rab11b and the protein sequence comparison provides a close look at all YPT3/Rab11-related genes cloned so far and clues to their possible relationship.