Nine strains of marine Proteobacteria were assayed for nucleic acid content during non-steady-state growth to assess whether a species-specific growth rate based on rRNA content is feasible for environmental samples. The large and small ribosomal subunits and genomic DNA were quantified using image analysis. It was found that the maximal intracellular concentration of 16S rRNA during batch growth for the bacteria averaged 155 fg+/-60 (S.D.) per cell for eight of the nine marine bacteria in the exponential phase (with the exception of one strain, Pac 218). The dilution/decay of 16S rRNA/cell was rapid with a return to pre-shift up values within 6-12 h for all strains except Vibrio fisherii. An overall relationship between the RNA:DNA ratio and the specific growth rate for non-steady-state growth for all bacterial strains was not observed as previously described for other Proteobacteria during steady-state growth. However, a predictable relationship between rRNA content and growth rate for many isolates during batch growth was observed. Furthermore, the rapid kinetics of intracellular rRNA levels indicates it will be feasible to assess whether specific bacteria are in steady state or non-steady state in the marine environment. If the condition of steady state is met for a specific Proteobacterial group in an environmental sample, it will be possible to estimate species-specific growth rates by measuring rRNA content.