Correlative microscopy was applied to study the influence of solids retention time on activated sludge floc structure. Conventional optical microscopy revealed flocs at lower SRTs (4 and 9 days) to be irregular in shape while flocs at higher SRTs (16 and 20 days) had a more spherical and compact structure. Flocs were examined by environmental scanning electron microscopy (ESEM) and by transmission electron microscopy (TEM) coupled with energy-dispersive spectroscopy (EDS). Distinctive differences in floc structure and the arrangement of EPS were revealed. Flocs from higher SRTs were less hydrated and were found to possess a dense EPS layer that covers much of the surface. Extracellular osmiophilic granules present in these flocs indicate that the cells at the higher SRT may produce more lipid-like material. This EPS layer appears to decrease the floc surface roughness and protects the interior cells from disruption by changes in the external environment. Sludge flocs at higher SRTs were found to be physically more stable than those at lower SRTs.