The genome of the model organism Drosophila melanogaster has been sequenced and annotated. Based on this groundwork, we performed a systematic genetic screen of the D. melanogaster X chromosome, which carries about one sixth of the genes of the organism. We generated a collection of single P-element insertions to provide genetic and molecular access to virtually all X-chromosomal genes. The study complements earlier work designed to systematically identify vital genes on the X chromosome by targeting transcription units which are phenotypically silent. We describe single UAS sequence-bearing P-element insertions throughout the X chromosome, which allows one to express the tagged genes under control of tissue/organ-directed GAL4 activity. In addition, the present collection of single insertion lines provides a tool to generate chromosomal deletions which are on average less than 33 kb in size.