In various activated sludge systems, sludge grows under transient (unbalanced) conditions and storage of internal polymers becomes important. Differences in storage capacity under transients are often used to explain kinetic control of bulking, but storage is neither studied in detail nor usually included in modelling. For this reason, the transient response of different aerobic mixed cultures was studied by experimentally determining the role of storage. Two different mixed cultures (bulking and non-bulking) were selected in an acetate-limited medium, by continuous or intermittent feeding of a CSTR, respectively. Batch tests were used to investigate the transient response of the selected cultures as a function of the starvation time and of the ratio of the initial concentration of the substrate and sludge biomass (So/Xo). In most experimental conditions, both cultures showed that the storage of poly-bydroxybutyrate (PHB) is in general the prevailing mechanism of substrate removal. In particular, the culture dominated by floc-formers showed very fast response to the substrate spike with a high observed yield. Storage was practically the only metabolism occurring. The ratio So/Xo did not have a major role in determining the type and extent of the response. Starvation did not affect the response of the floc-formers to transient conditions. For the filamentous bacteria, both the growth response and, even more significantly, the storage response were negatively affected. Hence, the difference in storage capacity between filamentous and floc-forming bacteria was further increased.