The cyanide dihydratases from Bacillus pumilus and Pseudomonas stutzeri share high amino acid sequence similarity throughout except for their highly divergent C-termini. However, deletion or exchange of the C-termini had different effects upon each enzyme. Here we extended previous studies and investigated how the C-terminus affects the activity and stability of three nitrilases, the cyanide dihydratases from B. pumilus (CynD(pum)) and P. stutzeri (CynD(stut)) and the cyanide hydratase from Neurospora crassa. Enzymes in which the C-terminal residues were deleted decreased in both activity and thermostability with increasing deletion lengths. However, CynD(stut) was more sensitive to such truncation than the other two enzymes. A domain of the P. stutzeri CynD(stut) C-terminus not found in the other enzymes, 306GERDST311, was shown to be necessary for functionality and explains the inactivity of the previously described CynD(stut-pum) hybrid. This suggests that the B. pumilus C-terminus, which lacks this motif, may have specific interactions elsewhere in the protein, preventing it from acting in trans on a heterologous CynD protein. We identify the dimerization interface A-surface region 195-206 (A2) from CynD(pum) as this interaction site. However, this A2 region did not rescue activity in C-terminally truncated CynD(stut)Delta 302 or enhance the activity of full-length CynD(stut) and therefore does not act as a general stability motif.