Comparative investigations on the inhibition of chitin synthetase of Mucor rouxii revealed that in contrast to results obtained in vivo, only few fungicides and other compounds inhibit the enzyme in vitro. Besides the well-known effect of polyoxin D, an inhibition was demonstrated for terrazol, tridemorph, hinosan, and formulated preparations of dimilin (PH 60--40, 60--38). The latter preparations, however, showed growth inhibition also with fungal species which do not synthesize chitin and the pure compounds are ineffective. Inhibition by phospholipase C, unsaturated fatty acids, and the reversibility of the inhibition caused by terrazol after addition of procain-hydrochloride demonstrates that phospholipids are essential for the activity of the enzyme, whereas sterols seem to be ineffective. Action of trypsin, PCNB, pentachlorophenol, and some similar compounds results in significantly increased activity, which in the case of trypsin could be due to the hydrolysis of a protein inhibitor. Hinosan inhibits the enzyme indirectly in a still unexplored manner.