At least twenty-seven proteins belong to the XylS/AraC family of prokaryote transcriptional regulators. All members of this family except CelD and TetD are positive transcriptional factors. Three subgroups were distinguished within the family in accordance with the Needleman and Wunsch algorithm. Multiple alignment of these proteins revealed that they shared a high degree of sequence homology at their C-terminal end, where a characteristic conserved motif, whose consensus sequence is I-DIA--GF-S--YF--F---G-TPS--R (where - means any aminoacid), was found. Within the homologous C-terminal region, but outside the above consensus motif, a putative DNA-binding domain organized as a helix-turn-helix motif was located in all regulators. For regulators recognizing chemical signals, the non-homologous N-terminal region of these regulators is presumed to contain binding sites for activator molecules that confer specificity.