Thailand has adjusted its production strategy by increasing the manufacturing of standard rubber or block rubber (Standard Thai Rubber or STR) in order to meet global market demand. About 90% of block rubber is STR20, for which the major raw material is cup coagulum. Consequently, demand for this raw material is increasing. The quality of cup coagulum produced by smallholders is so variable. Harvesting and post-harvest practices may influence the quality of cup coagulum. However, this has yet to be studied in Thailand. A study was therefore conducted in Surat Thani province to fill this gap. The methodology combined a survey of 79 farmers using a structured questionnaire to describe farmers' behaviour in cup coagulum production. Analysis of the physical and chemical properties of cup coagulum collected from a subsample of 26 farmers, as well as the properties of the dry rubber obtained from those cup coagula were also carried out. Cup coagula sampled from the 26 farmer subsample were characterised for foreign matter content. The dry rubber samples obtained from the processing of these collected cup coagula (creping-drying) were characterised for initial Wallace plasticity (P 0 ) and the plasticity retention index (PRI). We found that farmers' harvesting and post-harvest practices were very diverse. A classification of farmers based on these practices was proposed. The quality related parameters showed significant differences between farmers, with some of them being interestingly correlated with certain groups identified by the survey. This study confirmed that the quality of Thai natural rubber starts to be established much earlier than on the factory processing line producing STR bales. Smallholder behaviour may also be of great influence, and training/communication drives could improve the quality of the cup coagulum sold to factories and thereby help the Thai industry to continue producing excellent quality natural rubber.