1. An apparatus is described for testing substrate-depth preferences of Corophium volutator (Pallas).
2. Corophium burrow in mud whatever its depth, if only one depth of mud is available.
3. If presented with muds of different depths, more animals burrow in the deeper muds. Very shallow muds, of 0.5 cm. depth, are particularly avoided.
4. Although preferring deeper muds, animals are persuaded to burrow in shallow muds when the surface population density in the deeper muds is at a high level (> 0.1 animals/cm.2). This would be explained by a pattern of territorial behaviour which shows itself only at relatively high population densities.
5. Analysis of the distribution of animals at low densities indicates some tendency towards gregariousness.
6. Because of the periodic tendency for animals to vacate their burrows and burrow elsewhere, more than 50% of burrows at very low surface-population densities (ca. 0.01 animals/cm.2) may be unoccupied.
7. Larger animals tend to be found in deeper muds. This would lead to substrate depth influencing the age structure of populations in the field.
8. Limitations imposed by the shape and size of the burrow, rather than animal size in itself, probably explain why animals avoid shallow muds and why larger animals are found in relatively deeper muds.