Abstract: The microstructure of a collapsible loess was found to be dominated by the activity of worms to a depth of 26 m.The material was divided into dense and loose areas. The loose areas contained silt clay aggregates with large voids between them.The presence of large voids permitted collapse to occur. No significant cementation has been found so far, although some of the non collapsible material contained red materials.It is suggested that the parent material differs progressively up the profile.