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In past studies of enteric bacteria such as Escherichia coli, various measures of cellular RNA content have been shown to be strongly correlated with growth rate. We examined this correlation for four marine bacterial isolates. Isolates were grown in chemostats at four or five dilution rates, yielding growth rates that spanned the range typically determined for marine bacterial communities in nature (μ = 0.01 to 0.25 h^-1). All measures of RNA content (RNA cell^-1, RNA:biovolume ratio, RNA:DNA ratio, RNA:DNA:biovolume ratio) were significantly different among isolates. Normalizing RNA content to cell volume substantially reduced, but did not eliminate, these differences. On average, the correlation between μ and the RNA:DNA ratio accounted for 94% of variance when isolates were considered individually. For data pooled across isolates (analogous to an average measurement for a community), the ratio of RNA:DNA …