The work described in this paper was carried out in the summers of 1980 and 1981 in the western Irish sea. The purpose was to ascertain the effects of the annual thermal stratification which occurs in the region on the distribution of bacteria. The sampling regime involved a line of ten stations on a transect normal to a tidal mixing front, sampled at four depths in 1980 and six depths in 1981. Estimation of bacterial numbers was carried out by using a modified acridine orange epifluorescent staining method. The data was statistically analysed by using a nested analysis of variance technique which showed a stable and relatively uniform winter-early spring distribution with low overall counts which became more patchy as the spring progressed and the counts increased. In late summer the distribution became more uniform again within the mixed and stratified water masses but with significantly higher counts in the stratified water.