To examine the diversity of cultivable 0.2 micron filtrate biofilm forming bacteria from drinking water systems. Potable chlorinated drinking water hosts phylogenetically diverse ultramicrocells (UMC) (0.2 and 0.1 mu m filterable). UMC (starved or dwarf bacteria) were isolated by cultivation on minimal medium from a flow system wall model with polyvinyl chloride (PVC) pipes. All cultivated cells (25 different isolates) did not maintain their ultra-size after passages on rich media. Cultured UMC were identified by their 16S ribosomal DNA sequences. The results showed that they were closely related to uncultured and cultured members of the Proteobacteria, Actinobacteria and Firmicutes. The isolates of phylum Actinobacteria included representatives of a diverse set of Actinobacterial families: Micrococcaceae, Microbacteriaceae, Dermabacteraceae, Nocardiaceae and Nocardioidaceae. This study is the first to show an abundance of cultivable UMC of various phyla in drinking water system, including a high frequency of bacteria known to be involved in opportunistic infections, such as Stenotrophomonas maltophilia, Microbacterium sp., Pandoraea sp. and Afipia strains. Chlorinated tap water filtrate (0.2 and 0.1 mu m) still harbours opportunistic micro-organisms that can pose some health threat.