Three scattering layers exist in the minimum O2 region of the eastern tropical Pacific. Maximum scattering within these layers was determined by decreasing echosounder gain. Regression analyses were made of various biological and chemical parameters, and correlation coefficients between these parameters and maximum scattering depths were determined. The correlation coefficients show peak nitrite concentrations to be intimately related to the shallowest migrating scattering layer. In addition, correlations between peaks of scattering in the surface layer and maximum C14 activity and chlorophyll a concentration suggested that these parameters may possibly be measured by high-frequency sound sources. Trawl data suggest small bathypelagic fishes and zooplankton as causes of the shallowest migrating scattering layer and large bathypelagic fishes as the cause for the deepest layer.