Areas on different sides of the wharf of the Atlantic Biological Station that had been denuded in the spring and that had different degrees of illumination were selected and the growth of Balanus followed throughout the summer season. The results show (i) that there is a significant correspondence between rate of growth of Balanus and the amount of illumination, (ii) that the difference between the amount of growth at widely separated vertical levels is quite similar to the difference in the increment of growth in areas on the same level which are subject to wide variations in the amount of illumination, and (iii) that during the summer of 1931, there was no appreciable set of barnacles after June 6 in the St. Andrews region. The presence of a large number of elongated barnacles may result from the operation of one main factor or from the combination of several. It may indicate (i) that there has been a very heavy set, in which case exceptionally favorable growing conditions cannot necessarily be inferred, or (ii) a rapid rate of growth even though the set has not been particularly thick, or (iii) the presence of favorable growing conditions together with a good set. Each of these possibilities should be considered in the evaluation of Balanus balanoides as an organic indicator.