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We investigated the vertical distribution of Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa at four locations around the archipelago of Svalbard in autumn. The older and larger copepodites of Calanus spp. were generally located deeper in the water column. Differences in vertical distribution between stations partly reflected a south-north gradient in developmental progress with higher abundance of older stages in the southern locations. The C. finmarchicus and M. longa observations were consistent with the hypothesis that the developmental stages distributed according to certain preferences for light intensity, and different optical properties at the four locations are likely to have affected the vertical distributions. Diel vertical migration was only observed for older developmental stages of M. longa while young stages of M. longa remained in deep waters both day and night. A mortality index indicated that non-migrating Calanus spp. suffered higher mortality than migrating M. longa. [ABSTRACT FROM AUTHOR]