This is the first study, to my knowledge, quantifying the respiratory pigment haemoglobin discovered in a deep–sea copepod. Haemoglobin in copepods has previously been documented in only one other species from the deep water of an Italian lake. Specimens of the siphonostomatoid Scotoecetes introrsus Humes were collected during submersible dives at 2500 m depth near a hydrothermal vent at the East Pacific Rise (9°N). The haemoglobin content in the copepod’ haemolymph was 4.3 ± 0.6 μg per individual female (n = 6) and 1.8 ± 0.1 μg per individual male (n = 6). Weight–specific concentrations of haemoglobin were identical for females and males (0.25 ± 0.04 and 0.26 ± 0.02O NTμg per microgram dry weight, respectively). These haemoglobin concentrations are higher than those found in other small crustaceans. Activity of the electron transport system indicated that the respiration rates in S. introrsus (13.7 ± 7.7 μl O2 per milligram dry weight per hour) were similar to those in the shallow–water copepod Acartia tonsa (9.1 ± 1.3 μl O2 per milligram dry weight per hour). It was concluded that the possession of highly concentrated haemoglobin allows S. introrsus to colonize a geologically young, thermally active site such as the vicinity of a hydrothermal vent, despite the prevailing oxygen depletion.