Membrane　filtration　in　various　forms　has　become　an　increasingly　used　treatment　method　worldwide　for　the　supply　of　safe　drinking　water.　The　fouling　of　membranes　is　commonly　considered　to　be　the　major　operational　limitation　to　its　wider　application　since　it　leads　to　frequent　backwashing　and　a　shortening　of　membrane　life,　and　increased　production　costs.　The　components　of　natural　organic　matter　(NOM)　in　surface　waters　have　been　reported　previously　to　be　important　foulants　of　nanofiltration　(NF)　membranes,　however,　the　potential　beneficial　effect　of　particular　components　of　these　＇foulants＇　has　not　been　investigated　or　demonstrated　to　date.　In　this　study,　we　have　considered　the　roles　of　different　organic　materials　including　autochthonous　NOM　(e.g.,　biopolymers)　and　allochthonous　NOM　(e.g.,　humic　substances)　on　the　fouling　of　NF　membranes　by　bench-scale　tests　with　samples　of　two　representative　source　waters　(UK)　taken　in　two　different　seasons　(autumn　and　winter).　Microfiltration　(MF)　and　ultrafiltration　(UF)　were　employed　to　generate　two　permeates,　between　which　the　presence　of　biopolymers　(30　kDa　-　90　kDa)　is　the　major　difference.　We　developed　sequential　filtration　(MF/UFNF)　to　investigate　biopolymers＇　behaviours　in　NF　process.　The　results　showed　that　the　accumulation　of　biopolymers　on　NF　membranes　can　mitigate　fouling　by　providing　a　protective　layer　in　which　medium-low　molecular　weight　(MW)　materials　(e.g.　humic　substances)　are　separated　by　adsorption　and/or　size　exclusion.　The　protective　layers　assisted　by　biopolymers　were　seen　to　be　thicker　under　scanning　electron　microscope　(SEM)　observation　and　characterized　by　higher　roughness　(i.e.　three-dimensional,　spacial　structure)　and　greater　adsorptive　capacity.　Moreover,　improvement　on　NF　membrane　fouling　mitigation　could　be　more　significant　in　autumn,　comparing　to　that　in　winter.　The　findings　in　this　study　were　found　to　be　repeatable　in　similar　tests　with　samples　of　comparable　raw　waters　in　China,　and　will　be　important　to　the　practical　application　of　NF　membrane　systems　in　terms　of　a　new　approach　to　combating　fouling　in　long-term　operation.