15N-tracer experiments using natural populations of marine nitrifying bacteria revealed photoinhibition of nitrite- and ammonia-oxidizing activity at light intensities less than 1% of sunlight. Nitrite oxidation was more sensitive to light than was ammonia oxidation. Calculations of biologically effective dose rates suggest that the depths at which ammonia oxidation exceeds nitrite oxidation are consistent with the depths of observed primary nitrite maxima in subtropical waters.