ABSTRACT: As part of an investigation of ribonucleic acid (RNA) content as an index of growth and nutritional condition of zooplankton in the field, we describe here a method for measuring RNA and DNA in individuals, Stage N5 through adult, of the copepodCalanus finmarchicus. We used the technique to compare total RNA and DNA content and RNA:DNA ratios of C. finmarchicus copepodite stages cultured at different food densities. Copepods reared at growth-limiting phytoplankton concentrations(25 µg carbon l^-1) were smaller, had lower RNA:DNA ratios, and contained less total RNA and DNA than did copepods reared in excess food (500 µg C l^-1). This was true for each stage from C1 to C5. Stages C5 and C4 C.finmarchicus collected from Georges Bank and the Gulf of Maine were compared to those from the laboratory experiment. While RNA:DNA ratios of C5 and C4 individuals collected in May and June 1994 were intermediate between the 2 lab treatments, Stage C5C. finmarchicus collected in November 1993 had the lowest RNA:DNA ratios of all copepods sampled. This seasonality of RNA:DNA ratios is most likely related to food availability and changing metabolic activity. Our data show that RNA is a usefulindex of physiological condition for C. finmarchicus.