ABSTRACT: We investigated growth rate, nucleic acid (DNA, RNA) and protein indices and respiration in juvenile (8.5 to 12 mm total body length, 7 to 20 mg wet wt) and young adult (12 to 14 mm, 20 to 30 mg wet wt) Mysis relicta, as a function of temperature, body mass and molt stage in order to develop methods to assess condition or growth in the field. Mysids were exposed to either a preferred temperature (6.5°C) and 3 ration levels, or a range of constant and dielly-cycling (DC) temperatures with ad libitum feeding. Mysid growth parameters (specific rates of growth [SGR], respiration [MO₂], and RNA content cell^–1) integrated the DC temperature experienced as averaged responses weighted by the time spent at each temperature. MO₂ peaked at 12.7°C on acute temperature exposure from 4.2°C. MO₂ compensation with prolonged temperature exposure occurred at mean diel temperatures ≤8.5°C. Mysids could not survive at 16°C even for 5 h d^–1. These results confirm behavioral observations of temperature preferences. RNA concentration in M. relicta increased with ration and decreasing temperatures. Protein:DNA ratio, %protein and SGR increased with ration and then plateaued. Protein:DNA ratio, %protein and DNA:weight ratio did not change with temperature with unlimited feeding. Forward, stepwise, multiple regression models for each experiment and the combined data accounted for 31 to 72% of variability in SGR. Our experimental data provide guidance, a preliminary temperature-correction factor for RNA, and benchmarks for use of nucleic acid and protein indices in assessing growth or condition of M. relicta in the field.