The RNA polymerase II (RNAP II) largest subunit contains a C-terminal domain (CTD) with up to 52 Tyr(1)-Ser(2)-Pro(3)-Thr(4)-Ser(5)-Pro(6)-Ser(7) consensus repeats. Serines 2, 5, and 7 are known to be phosphorylated, and these modifications help to orchestrate the interplay between transcription and processing of messenger RNA (mRNA) precursors. Here, we provide evidence that phosphorylation of CTD Thr(4) residues is required specifically for histone mRNA 3' end processing, functioning to facilitate recruitment of 3' processing factors to histone genes. Like Ser(2), Thr(4) phosphorylation requires the CTD kinase CDK9 and is evolutionarily conserved from yeast to human. Our data thus illustrate how a CTD modification can play a highly specific role in facilitating efficient gene expression.