The determined state of Drosophila imaginal discs depends on stable patterns of homeotic gene expression. The stability of these patterns requires the function of the ash1 gene, a member of the trithorax group. The primary translation product of the 7.5-kb ash1 transcript is predicted to be a basic protein of 2144 amino acids. The ASHl protein contains a SET domain and a PHD finger. Both of these motifs are found in the products of some trithorax group and Polycomb group genes. We have determined the nucleotide sequence alterations in 10 ash1 mutant alleles and have examined their mutant phenotype. The best candidate for a null allele is ashl  ²². The truncated protein product of this mutant allele is predicted to contain only 47 amino acids. The ASHl protein is localized on polytene chromosomes of larval salivary glands at >100 sites. The chromosomal localization of ASHl implies that it functions at the transcriptional level to maintain the expression pattern of homeotic selector genes.