An electron density map at 5.5 Å resolution has been computed for γ-chymotrypsin inhibited with toluenesulfonyl fluoride. Data were collected for the native enzyme crystals, for the inhibited enzyme tosyl-γCHT† † Abbreviations used: CHT, chymotrypsin; tosyl-F, toluenesulfonyl fluoride; pipsyl-F, p-iodobenzenesulfonyl fluoride; PMMCMBS-, p-methoxy-m-chloromercuribenzenesulfonyl-; PMS-, phenylmethanesulfonyl. and for four heavy-atom derivatives prepared by soaking the crystals in p-iodophenylsulfonyl fluoride, p-methoxy-m-chloromercuribenzenesulfonyl fluoride, KI3 and K2HgI4. The shape of the molecule has been determined and the position of the active center located. The γCHT molecule has been compared with αCHT. © 1969.