Mud samples from aquatic environments in many parts of Great Britain and Ireland were collected, mainly in 1975 and 1976, and examined for Clostridium botulinum. The samples were taken from lakes, ponds, reservoirs, marshes, mudflats, streams, rivers and canals, and the sampling areas included a number of bird refuges and reserves. Of 554 samples 194 (35.0%) were positive and 167 (30.1%), 19 (3.4%), 6 (1.1%) and 15 (2.7%) contained types B, C, D and E respectively; 13 (2.3%) contained more than one type. Each type demonstrated was found in both fresh-and salt-water environments. Type B was widespread; types C, D and E were demonstrated in widely separated areas in England and Wales, and type C was found in both the north and south of Scotland. The results were compared with those reported earlier in respect of surveys in the London area, the Norfolk Boads and the Camargue (France).