Genetic and biochemical studies have identified many factors thought to be important for transcription elongation. We investigated relationships between three classes of these factors: (1) transcription elongation factors Spt4-Spt5, TFIIS, and Spt16; (2) the C-terminal heptapeptide repeat domain (CTD) of RNA polymerase II; and (3) protein kinases that phosphorylate the CTD and a phosphatase that dephosphorylates it. We observe that spt4 and spt5 mutations cause strong synthetic phenotypes in combination with mutations that shorten or alter the composition of the CTD; affect the Kin28, Bur1, or Ctk1 CTD kinases; and affect the CTD phosphatase Fcp1. We show that Spt5 co-immunoprecipitates with RNA polymerase II that has either a hyper- or a hypophosphorylated CTD. Furthermore, mutation of the CTD or of CTD modifying enzymes does not affect the ability of Spt5 to bind RNA polymerase II. We find a similar set of genetic interactions between the CTD, CTD modifying enzymes, and TFIIS. In contrast, an spt16 mutation did not show these interactions. These results suggest that the CTD plays a key role in modulating elongation in vivo and that at least a subset of elongation factors are dependent upon the CTD for their normal function.