3′ Untranslated region processing and polyadenylation in Trichomonas vaginalis was analyzed by 3′ rapid amplification of cDNA ends and sequence analysis of T. vaginalis mRNAs. A putative polyadenylation signal with the sequence UAAA was found 11–30 nucleotides upstream from the cleavage site. The motif pyrimidine↓(A)0–3AAUU is proposed to be the cleavage site for polyadenylation of transcripts. This potential sequence defining the cleavage site for polyadenylation in eukaryotes is a novel finding. As in other eukaryotes, runs of several U's downstream from the cleavage site were identified. A working hypothesis is proposed which couples the UAA translation stop codon with the signaling for the 3′end processing of transcripts in this early divergent parasitic protozoa.