Considering the ecological importance and potential value ofHediste diversicolor, abetter understanding of its metabolic rate and potential growth rates is required.The aims of this study are: (i) to describe key biometric relationships; (ii) to test theeffects of temperature and body size on standard metabolic rate (as measure by oxygenconsumption) to determine critical parameters, namely Arrhenius temperature (TA),allometric coefficient (b) and reaction rate; and (iii) to determine the metabolic scopefor activity (MSA) ofH. diversicolorfor further comparison with published specificgrowth rates. Individuals were collected in a Mediterranean lagoon (France). After 10days of acclimatization, 7 days at a fixed temperature and 24 h of fasting, resting oxygenconsumption rates(VO2) were individually measured in the dark at four differenttemperatures (11, 17, 22 and 27◦C) in worms weighing from 4 to 94 mgDW (n=27 pertemperature). Results showed that DW and L3 were the most accurate measurements ofweight and length, respectively, among all the metrics tested. Conversion of WW (mg),DW (mg) and L3 (mm) were quantified with the following equations: DW=0.15×WW, L3=0.025×TL(mm)+1.44 and DW=0.8×L33.68. Using an equation basedon temperature and allometric effects, the allometric coefficient (b) was estimated at 0.8for DW and at 2.83 for L3. The reaction rate (VO2) equaled to 12.33μmol gDW−1h−1and 0.05μmol mm L3−1h−1at the reference temperature (20◦C, 293.15 K). Arrheniustemperature (TA) was 5,707 and 5,664 K (for DW and L3, respectively). Metabolic scopefor activity ranged from 120.1 to 627.6 J gDW−1d−1. Predicted maximum growth rateincreased with temperature, with expected values of 7–10% in the range of 15–20◦C.MSA was then used to evaluate specific growth rates (SGR) in several experiments. Thispaper may be used as a reference and could have interesting applications in the fieldsof aquaculture, ecology and biogeochemical processes.