The fluorophore [2-(4-pyridyl)-5{[4-dimethylaminoethyl-aminocarbamoyl-methoxy]phenyl}oxazole], in short PDMPO, is incorporated in newly polymerized silica in diatom frustules and thereby provides a tool to estimate Si uptake, study diatom cell cycles but also determine mortality-independent abundance-based species specificgrowth rates in cultures and natural assemblages. In this study, the theoretical framework and applicability of the PDMPO staining technique to estimate diatom species specific-growth rates were investigated. Three common polar diatom species, Pseudo-nitzschia subcurvata, Chaetoceros simplex, and Thalassiosira sp., chosen in order to cover a broad range of species specific frustule and life-cycle characteristics, were incubated over 24 h in control (no PDMPO) and with 0.125 and 0.6 μM PDMPO addition, respectively. Results indicate that specific-growth rates of the species tested were not affected in both treatments with PDMPO addition. The specific-growth rate estimates based on the PDMPO staining patterns (μ PDMPO ) were comparable and more robust than growth rates estimated from the changes in cell concentrations (μ cc) . This technique also allowed to investigate and highlight the importance of the illumination cycle (light and dark phases) on cell division in diatoms.