This paper examines the morphological characteristics of Microthrix parvicella grown under conditions which approximate to those that can occur in domestic sewage treatment systems. The results show that the filaments display both a variable morphology and variable Gram stain reactions when fed on selective substrate (Tween 80). Long Gram-positive filaments broke down within a few days into short Gram-negative filaments and single cells with Neisser-positive granules. This change was observed in certain sequences. The data are reviewed along with examples of other filaments which display variable morphology in pure and natural culture, along with the implications that polymorphism has on the management of separation problems in the activated sludge process.