Choline acetyl transferase (EC 220.127.116.11) (ChAT) present in cholinergic nerve endings catalyzes the synthesis of acetyl choline (Ach) from choline and acetylCoA. The membrane-bound form (integral membrane protein) of the choline acetyl transferase was prepared from squid head ganglia. The preparation procedure includes repeated hypo-osmotic disruption and washing by ultracentrifugation in a high ionic strength buffer. It appears that approximately 20\% of the synaptosomal choline acetyl transferase is present as a integral protein, non-ionically bound to membrane. It was found that at high concentrations, the acetylcholine(Ach) inhibits soluble ChAT to a greater degree than the membrane-bound enzyme form. The activities of soluble and membrane-bound ChAT also decrease with increasing calcium concentration but the activity of soluble ChAT is inhibited less than that of membrane-bound ChAT. The activity of soluble ChAT is completely lost at 2.0 mM SDS concentration whereas the membrane-bound ChAT still exhibits about 74\% of the control value at 3.0 mM SDS concentration. Soluble ChAT exhibits a sharp pH optimum near 6.6 in potassium phosphat buffer wheas membrane-bound ChAT shows a broader pH-activity profile. Optimum temperature of both soluble and membrane-bound forms of ChAT is $32-35\,^\circ\!C$ but over the range of $35-56\,^\circ\!C$, the membrane-bound ChAT is more stable than the soluble enzyme. Soluble and membrane-bound forms of ChAT exhibit identical Km values of approximately 0.12 mM for the substrate of acetyl-CoA. These results strongly indicate that two forms of ChAT, one soluble and one membrane-bound, are present in squid head ganglia.