The increasing use of surface waters for municipal water supplies has increased the importance of plankton counting and enumerating in determining water quality. Attention accordingly is focused on methods employed by laboratories which routinely count or otherwise analyze plankton, since the accuracy of any result depends to a large extent on the methodology employed, as well as on the training and skill of the analyst. This study was conducted to determine the variations that may occur among the final results when many laboratories analyze alliquots of the same sample, and the extent to which the method of analysis affects the results obtained. Both qualitative and quantitative aspects were considered.