Underwater objects always acquire a growth of marine organisms when left in the water for a period of time. When a natural growth of fouling occurs on a hydrophone, a reduction of its sensitivity and a deterioration of its beam pattern may be expected. In order to estimate the severity of these effects, a small selection of hydrophones was measured after they had remained in Chesapeake Bay during the growing season of 1961. Measurements of beam pattern and receiving response were made in the fouled condition and after the fouling had been cleaned off. Reductions of axial sensitivity ranging from zero to 10 dB were found in the frequency interval 1 to 20 kc. Appreciable, though not severe, effects of the fouling on the hydrophone beam patterns were observed. Additional work is needed for making prediction estimates for surveillance systems or other uses.