1. The data for five planktonic taxa sampled during the SOND cruise (autumn 1965) by R.R.S. ‘Discovery’ in a day and a night series of horizontally towed nets have been analysed by factor and cluster analyses.
2. The factor analyses of hauls showed that there were five biologically distinct zones in the water column between the surface and 1000 m. The zones were more distinct in the day series than in the night.
3. Factor analyses of the individual taxa gave sensible groupings of species and the factor score plots gave ‘averaged’ depth distributions for each group.
4. Cluster analyses of the individual taxa gave results consistent with the factor analyses. Both methods gave groupings which fitted the zonation patterns of the hauls, except for the two zones between 100 and 460 m at night.
5. Only cluster analyses could be carried out on the total data matrices. Although satisfactory interpretation was only possible with the aid of the analyses of the individual taxa, the zonation and species groupings were again largely retained.
6. The zonation of the planktonic taxa is shown to be very similar to that described for nektonic species sampled on the same cruise.
7. The usefulness of these analytical methods is compared with the conclusions of other investigators. It is concluded that the rotation of the matrices to simple structure in the factor analysis gave a marked improvement in the ease of interpretation.
8. For this data, principal component analyses gave very similar results to the full factor analyses.