Polyclonal anti-chitin synthase antibodies raised against the Saccharomyces cerevisiae CHS2 gene product were used to identify and localize chitin synthase in the filamentous ascomycete Neumspora crassa. A single band of approximately 11 0 kDa was observed in Western blots of total protein extracts of N. crassa, probed with these antibodies. However, several additional bands were labelled when membrane fraction proteins (microsomes) were probed. Histo-immunochemical localization of chitin synthase confirmed that the polypeptide is compartmentalized in membranous vesicles (chitosomes), which are abundant in the vicinity of the hyphal tip. TEM analysis did not reveal chitin synthase in the plasma membrane. However, dense labelling of membrane-associated chitin synthase was observed by light-microscopic analysis of N. crassa protoplasts and at young hyphal tips.