The pattern of the infauna on particulate shores of the southern coasts of the Severn Estuary has been examined. Although mud predominates as the major beach substratum within the Estuary, sand beaches occur in both the lower and the upper reaches. Liquid mud is found at l.w. in the middle reaches. Suitability of substratum is shown to be the most important factor influencing animal distribution. The number of infaunal species found in this survey did not increase markedly towards the mouth, in contrast to the pattern displayed by rocky shore animals. Several species were rare or absent on mud beaches close to the entrance of the River Avon and the possibility that sewage contamination is responsible for this is discussed. The methods of sampling differed slightly at different transect sites. Normally one quadrat of area 0.25m2 was taken at each station and sieved through a mesh of 0.5mm. When the substratum was very firm mud or clay with a surface coating of softer mud which was less than 10cm deep, this surface layer only was taken, since it was impossible to force the clay through the sieve. Such samples were supplemented with 10cm deep cores of area 0.01m2. No distinction was made in the reprint of which stations had which method of sampling on them so both methods have been recorded at each station.