Oithonaspp. is considered the most abundant and ubiquitous copepod genus in the marine environment, often outnumbering calanoid copepods throughout the year. Previous studies have argued that one of the reasons for such success is that the respiration rate ofOithonaspp. is insensitive to temperature changes and lower than in calanoids. However, comprehensive data on the thermal biology of this important copepod genus is lacking. In this study, the respiration rate of adult femaleO. similisfrom the English Channel, was measured over the temperature range 4 to 25°C. The respiration rate ofO. similischanged exponentially with temperature (ln O₂-rate = –3.59 + 0.114T, df = 35, r² = 0.85, p < 0.001,Q 10= 3.1) similar to that of other poikilotherms. Over the temperature range examined,O. similisbasic metabolic cost varied from a minimum of ~1.4% body-C d–1at 4°C to a maximum of 23% body-C d–1at 25°C, corresponding to an energy demand of ~3% and 32% body-C d–1respectively. The respiration rate ofO. similis, from the present study, is ~8 times lower than that of a calanoid copepod of equivalent body weight estimated from published empirical metabolism–temperature data. We suggest that these differences in metabolic rates may account for the year-round persistence and higher abundances ofOithonaspp. over calanoid copepods, particularly in oceanic and oligotrophic environments where food resources may be limiting for calanoid copepods.