The Eikelboom filamentous morphotype 0803 is commonly found in activated sludge systems globally, where it contributes to sludge bulking events. Earlier reports have suggested that it is a member of both the Proteobacteria and Chloroflexi. This study shows that this filament contributing to a period of poor sludge settleability in an Australian activated sludge plant is a member of the Chloroflexi, but not within the Caldilinea, as reported for this morphotype in Danish plants. Instead, it is a member of the Anaerolineae. The fluorescent signals generated in these filaments using the FISH probes designed here were unevenly distributed, a situation similar to that seen earlier in the Anaerolineae morphotype 0092 to which it is more closely related phylogenetically than it is to the Caldilinea morphotype 0803. FISH-based surveys showed that this 0803 phylotype is uncommon in Australian activated sludge systems, and where seen is present usually at low abundances. The FISH probes described here will facilitate attempts to map the distribution and impact of this Australian filament morphotype 0803 in activated sludge systems of different configurations in plants around the world.