Scaphocalanus magnus is a large copepod species with a worldwide distribution. Information on the ecology of this species is scarce. In this study, S. magnus was collected using vertically stratified net sampling from an ice station in the western Arctic Ocean over a year. The vertical distribution differed between periods of polar night and midnight sun, with core depth distributions of 264–381 m for the polar night and 518–745 m for the midnight sun. The shallower distribution during the polar night may have resulted from the animals moving upwards to find sufficient food during the less productive season. The abundance of early copepodite stages showed clear seasonality. The C2 and C3 stages were abundant June–August and August–September, respectively. After C4, seasonality in abundance was not clear. For C4 and C5, body sizes and masses were greater for males than for females while the opposite was the case for C6. Adult males do not feed and thus have shorter life spans than females, skewing the adult sex ratio towards females. For these mesopelagic particle feeding copepods, a long residence time at C6F suggests that in a varying food environment, it is possible to wait to initiate reproduction until favourable food conditions occur.