The common fruitfly, Drosophila, is the most extensively studied of all organisms from the standpoint of genetics and cytology. This atlas summarizes what is known about the approximately 100 Drosophila genes for which the complete nucleotide sequence is known. Each entry includes a description of the gene's molecular organization and expression, the complete nucleotide and amino acid sequences, maps of interesting structures, highlights of functional features and promoter regulatory regions, and selected references to the primary literature. A separate section of the atlas considers different aspects of gene organization as they occur in the Drosophila genome. Topics covered include size correlations among various genetic elements, splicing signals, translation initiation signals, and codon bias. The work represents a new milestone in summarizing current information and making it easily accessible to geneticists and biologists.