For enhancing the survival of genetically engineered microorganisms (GEMs) in activated sludge processes, the use of a floc-forming bacterium as the host for a recombinant plasmid was proposed. The hoc-forming and phenol-degrading GEM Sphingomonas paucimobilis 551 (pS10-45) was cultured to demonstrate this proposal. Although the maximum growth rate of the host strain S. paucimobilis 551 was low and the recombinant plasmid pS10-45 was unstable in the host, the resultant GEM S. paucimobilis 551 (pS10-45) was difficult to wash out together with the effluent, and it maintained population 3-4 times higher than the non-hoc-forming GEM Escherichia coli HB101 (pS10-45) in a model activated sludge process operated in a sequencing batch mode. In the long run, the GEM-inoculated activated sludge process showed better phenol removal ability by the recombinant plasmid and better sludge settlement by the host strain.