False Bay, a purely marine bay over 400 sq. miles in area with maximal depth of 90 m., is affected by the warm Agulhas Current and the cold Benguela Current. Although a limited number of samples of soft substrata showed no correlation of average particle size or of degree of sorting with depth, the content of organic matter ‘available’ as food increased with depth from 35 m. downwards. The biotas of rocky and soft bottoms were sampled by dredging and soft bottoms additionally by diving. The characteristically different biotas of such substrata were well recognized despite different amenability to sampling methods. Rocky biotas showed a broad pattern of vertical zonation when the optimal ranges of the commonest species were considered but consideration of the extreme ranges of these species showed that there was progressive thinning of the rich and varied biota of the shallows as depth increased without augmentation by species characteristic of deep rocks alone. Although soft-bottom biotas showed indisputable changes with depth, such as a change from shallow- to deep-shelf biota at about 80 m. and the relative barrenness shallower than 10 m. in regions beneath strong swell, they are more obviously arranged into grounds, some of which are tentatively described. But analysis of the extreme ranges of depth distribution of the commonest species shows a progressive replacement of shallow water species by those characteristic of deeper bottoms, a feature that contrasts sharply with that of the distribution of rocky biotas.