Nowadays the excessive growth of many filamentous bacteria in activated sludge plants can be controlled by the application of so-called selectors. In the Netherlands, about 80 selectors have been constructed at full scale plants so far. However, selectors are frequently not very effective for controlling bulking if M. parvicella is dominating the sludge population. As M. parvicella has also been frequently observed in nutrient removal plants, the development of control strategies for this bacterium has become very urgent. From the substantial information on the occurrence of and attempts to control M. parvicella collected during the last decade, several main conclusions can be drawn. The development of M. parvicella in Dutch treatment plants shows a very distinctive seasonal rhythm, the population reaching its maximum size in early spring. Foaming in Dutch treatment plants is usually caused by M. parvicella. M. parvicella and Actinomycetes spp. seem to compete for the same ecological niche in treatment plants. By comparing the population size of M. parvicella as a function of various modifications in the activated sludge process, it is concluded that this bacterium grows better in so-called carrousel systems than in other extended aeration plants. M. parvicella causes severe bulking in carrousels fed with presettled sewage. The impact/usefulness of selectors for controlling M. parvicella seems to decrease as the overall sludge load of the plant increases. This information is presented to start a discussion about possibilities to solve the M. parvicella puzzle.