The objective of the research was to obtain insights into the behavior of microorganisms under feast/famine conditions as often occur in wastewater treatment processes. The response of microorganisms to such conditions is the accumulation of storage polymers like poly(β-hydroxybutyrate). The research was performed using a pure culture of Paracoccus pantotrophus LMD 94.21. A steady-state C-limited chemostat culture was switched to batch mode and a pulse of acetate was added. As long as external substrate (acetic acid) was present, the organism grew and accumulated poly(β-hydroxybutyrate). After depletion of the external substrate, the stored poly(β-hydroxybutyrate) was used as growth substrate. Poly(β-hydroxybutyrate) accumulation was found to be strongly dependent on the growth rate of the organism before the pulse addition of acetate. Poly(β-hydroxybutyrate) accumulation was correlated to the difference in maximum acetate uptake rate and the acetate required for growth. Based on the interpretation of the experimental results, a metabolically structured model has been set up. This model adequately describes the observed kinetics of the poly(β-hydroxybutyrate) formation and consumption. © 1997 John Wiley & Sons, Inc. Biotechnol Bioeng55: 773–782, 1997.