Insect remains, chiefly of beetles, have been obtained from a Late Pleistocene organic mud which was interbedded within the Middle Sands at Chelford, Cheshire. These deposits represent an interstadial during an early phase of the Last (Würm) Glaciation. The insects were usually preserved as fragments embedded in felted vegetable matter but associated pieces representing almost complete animals were occasionally found. Remarkably good preservation was not uncommon and structural colours, scales and hairs were often retained. It has been possible to match most of the remains with modern species but a few specimens, though very distinctive, have so far defied all attempts at specific identification and may represent extinct forms. A faunal list is provided and the relative abundance of each species is given. From a consideration of the modern environmental requirements of this fauna it has been possible to deduce much of the ecology of the district at the time of the deposition of the organic mud. It is suggested that this deposit was laid down in a stagnant, acid water pool choked with vegetable matter and partially overgrown by Sphagnum. The dominant trees in the district were conifers and birches with willows in the marshy ground near to the pool. The climate was cold but by no means extremely severe and it is suggested that the nearest equivalent may be found to-day in Finland between latitudes 60° and 64°. These deductions are discussed in the light of palaeobotanical evidence provided by the organic mud. The geological significance of the deposit is briefly discussed.