We report the complete sequence of the gene encoding mouse glial fibrillary acidic protein (GFAP), the intermediate filament (IF) protein specific to astrocytes. The 9.8 kb gene includes nine exons separated by introns ranging in size from 0.2 to 2.5 kb. A comparison of the organization of the GFAP gene with that of genes encoding other IF proteins reveals that the structure of IF genes is highly conserved in spite of considerable divergence at the amino acid level. Thus, most of the evolutionary events leading to the placement of introns in IF genes must have occurred prior to the duplication and subsequent divergence of IF genes from a presumptive common ancestral sequence. The conserved gene organization is unrelated to structural features of IF proteins. A curious feature of the GFAP gene is the large number of repeated sequences found in the introns. Six tracts of reiterated di- or trinucleotides are present, plus tandem repeats of two different novel sequences. One repeat is unique to the GFAP gene; the other occurs elsewhere in the mouse genome, although at relatively low frequency.