Chitin,　the　most　abundant　aminopolysaccharide　in　nature,　is　an　extracellular　polymer　consisting　of　N-acetylglucosamine　(GlcNAc)　units(1).　The　key　reactions　of　chitin　biosynthesis　are　catalysed　by　chitin　synthase(2-4),　a　membrane-integrated　glycosyltransferase　that　transfers　GlcNAc　from　UDP-GlcNAc　to　a　growing　chitin　chain.　However,　the　precise　mechanism　of　this　process　has　yet　to　be　elucidated.　Here　we　report　five　cryo-electron　microscopy　structures　of　a　chitin　synthase　from　the　devastating　soybean　root　rot　pathogenic　oomycete　Phytophthora　sojae　(PsChs1).　They　represent　the　apo,　GlcNAc-bound,　nascent　chitin　oligomer-bound,　UDP-bound　(post-synthesis)　and　chitin　synthase　inhibitor　nikkomycin　Z-bound　states　of　the　enzyme,　providing　detailed　views　into　the　multiple　steps　of　chitin　biosynthesis　and　its　competitive　inhibition.　The　structures　reveal　the　chitin　synthesis　reaction　chamber　that　has　the　substrate-binding　site,　the　catalytic　centre　and　the　entrance　to　the　polymer-translocating　channel　that　allows　the　product　polymer　to　be　discharged.　This　arrangement　reflects　consecutive　key　events　in　chitin　biosynthesis　from　UDP-GlcNAc　binding　and　polymer　elongation　to　the　release　of　the　product.　We　identified　a　swinging　loop　within　the　chitin-translocating　channel,　which　acts　as　a　＇gate　lock＇　that　prevents　the　substrate　from　leaving　while　directing　the　product　polymer　into　the　translocating　channel　for　discharge　to　the　extracellular　side　of　the　cell　membrane.　This　work　reveals　the　directional　multistep　mechanism　of　chitin　biosynthesis　and　provides　a　structural　basis　for　inhibition　of　chitin　synthesis.