The storage of poly-beta-hydroxybutyrate (PHB) in extended filamentous bacteria from activated sludge was monitored by Sudan Black staining: PHB granules were blue in the reddish filaments counterstained by safranin. By quantitative image analysis of colour images grabbed on an optical microscope, the distribution of the PHB loading of the extended filaments was estimated by determination of the proportion of blue pixels of their skeleton. The method was applied for different feed compositions to demonstrate its ability to monitor the PHB synthesis and storage capacity of filamentous bacteria in mixed cultures. Fast PHB storage, within a few hours, could be observed with acetate-based feeding solutions but the storage rate decreased with more complex feeds (meat extract based feed, wastewater).