The use of microbiocides, particularly chlorine may be advantageous to operation but can also exacerbate biofouling problems. Micro-organisms subjected to low levels of biocides often exude large amounts of extracellular polysaccharides (EPS) as a protection, and it is this EPS material that forms the biofilm. This paper examines the causes and effects of obstinate biofilms in membrane elements. In these cases problems of increased differential pressure have proven difficult to correct during routine cleaning cycles. Consequently, regrowth rates, as indicated by this differential pressure for such biofilms have been rapid. Experimental data has been taken from more than a hundred membrane autopsies from around the world. These autopsies have been undertaken when such problematic biofilms are encountered. These confirm that biofilms formed in process and membrane systems are comparable except that membrane biofilms contain greater numbers of fungi. Some operating systems have reported the cessation of chlorination with a significant reduction of biofouling. The paper considers possible causes for this. It also considers the increasing use of proprietary non-oxidising microbiocides. Conclusions are that biofouling is endemic within membrane systems, yet many systems operate satisfactorily even with a biofilm. Foulant layers can be ＇conditioned＇ or ＇hardened＇ by the repeated use of cleaning programmes and there is a strong case for alternating cleaners and biocides as used by the cooling water treatment industry.