The minimum size for splicing of a vertebrate intron is approximately 70 nucleotides. In Drosophila melanogaster, more than half of the introns are significantly below this minimum yet function well. Such short introns often lack the pyrimidine tract located between the branch point and 3' splice site common to metazoan introns. To investigate if small introns contain special sequences that facilitate their recognition, the sequences and factors required for the splicing of a 59-nucleotide intron from the D. melanogaster mle gene have been examined. This intron contains only a minimal region of interrupted pyrimidines downstream of the branch point. Instead, two longer, uninterrupted C-rich tracts are located between the 5' splice site and branch point. Both of these sequences are required for maximal in vivo and in vitro splicing. The upstream sequences are also required for maximal binding of factors to the 5' splice site, cross-linking of U2AF to precursor RNA, and assembly of the active spliceosome, suggesting that sequences upstream of the branch point influence events at both ends of the small mle intron. Thus, a very short intron lacking a classical pyrimidine tract between the branch point and 3' splice site requires accessory pyrimidine sequences in the short region between the 5' splice site and branch point.