Meiobenthos, here defined as metazoans passing through a ½ mm screen, is described from core samples collected on muddy grounds at 100-140 m depth in the North Sea and off the west coast of Scotland between February 1962 and August 1963.

Sixteen cores of 2·2 cm diameter were collected from Loch Nevis, and 14 from the Fladen ground. The top 7 cm of each core was passed through a 500μ then a 76μ screen. The residues were examined entire, and the nitrate of the finest mesh was subsampled.

The fauna consisted mainly of nematodes, kinorhynchs, ostracods, copepods, polychaetes, and lamellibranchs. Between 61 and 97 % of the population consisted of nematodes, but copepods were second in importance and were more abundant than recorded on other subtidal grounds.

The fine mesh (76μ) retained almost all specimens of every animal group except Nematoda. Numerically 38% of the nematode population passed through the fine sieve, but this represented only 18% by weight.

The mean dry weight of meiobenthos on Fladen was calculated as 0·76 g/m2 and the average number of individuals was 196 × 104/m2.

In Loch Nevis the corresponding weight was 1·28 g and the number 101 × 104. The loch had a larger and more varied population of all groups except nematodes, which were particularly abundant on Fladen.

The data do not indicate significant population fluctuations throughout the year in the permanent meiobenthos.

Compared with other subtidal grounds Nevis and Fladen seem to support much richer populations, and the high values for Fladen are of particular interest since this is a poor macrobenthos area.

An attempt is made to indicate the possible level of meiobenthos productivity and it is suggested that there may be competition between meio- and macrobenthos for available organic matter which could result in a less efficient production of fish food.