The Spt4, Spt5, and Spt6 proteins are conserved throughout eukaryotes and are believed to play critical and related roles in transcription. They have a positive role in transcription elongation inSaccharomyces cerevisiae and in the activation of transcription by the HIV Tat protein in human cells. In contrast, a complex of Spt4 and Spt5 is required in vitro for the inhibition of RNA polymerase II (Pol II) elongation by the drug DRB, suggesting also a negative role in vivo. To learn more about the function of the Spt4/Spt5 complex and Spt6 in vivo, we have identified Drosophila homologs of Spt5 and Spt6 and characterized their localization onDrosophila polytene chromosomes. We find that Spt5 and Spt6 localize extensively with the phosphorylated, actively elongating form of Pol II, to transcriptionally active sites during salivary gland development and upon heat shock. Furthermore, Spt5 and Spt6 do not colocalize widely with the unphosphorylated, nonelongating form of Pol II. These results strongly suggest that Spt5 and Spt6 play closely related roles associated with active transcription in vivo.