The number of bacteria in the fleece of Corriedale sheep has been shown to vary with rainfall. A significant difference has also been found between the numbers of bacteria in the fleece of sheep which have been selected as immune and susceptible, respectively, to bacterial discoloration. After heavy rain no difference could be found in the numbers of bacteria in the two fleece types, but susceptible fleeces tended to have a greater and less variable number of bacteria than immune fleeces over the subsequent drier periods.