Continuous-flow laboratory-scale activated sludge units were operated on domestic sewage at low F/M. In CSTR units at the F/M range of 0.05–0.25 g COD removed/g TMLVSS, day bulking did not occur with a weak (BOD5 = 139 mg/L) sewage feed and TMLSS = 1.5 g/L. Supplementation of sewage by blending with raw sludge produced a stronger sewage (BOD5 = 315 mg/L) which caused TMLSS to increase to 3.5 g/L. Bulking occurred in CSTR, 2, 4, 8, and 16 compartment units. An aeration basin with initial compartment l/32nd of aeration basin volume prevented but did not cure bulking. An aeration basin with initial compartments 1/74th of aeration basin volume, prevented and cured bulking. Anoxia in an aeration basin with two initial compartments each 1/64th of the total aeration basin volume did not aid in curing bulking. The SVI of sludges at low F/M (0.3–0.35 g COD removed/g TMLVSS, day) and high TMLSS (3.5 g/L) is related to conditions in the initial compartment rather than those in the remainder of the aeration basin. Initial compartment soluble COD, first-compartment F/M, and initial compartment size are important; floc loading, dispersion number and total number of aeration basin compartments are not important.