A strong secondary nitrite maximum was observed at 15 ¿S off the coast of Peru in August 1976. Since the secondary nitrite maximum does not coincide with a maximum in stability where sinking particulate organic material would accumulate, the hypothesis that the depth of nitrite accumulation is determined by the concentration of organic substrates on a density discontinuity is not tenable. Instead, the nitrite maximum coincides with a stability minimum within the core of the southward-moving Equatorial Subsurface Water in the Peru-Chile Undercurrent.