Two distinct kinds of particle maxima (nepheloid layers) were observed off Oregon in November 1977 and off Washington in October 1978 by an in situ light transmissometer: one in the thermocline in the euphotic zone and the other at intermediate depth well below the thermocline. The thermocline nepheloid layer is associated with well-defined maxima of dissolved oxygen, chlorophyll-a, and phaeophytin, and these associations suggest that the nepheloid layer is primarily composed of phytoplankton undergoing active photosynthesis. The intermediate nepheloid layer is found in connection with the bottom waters near the shelf break and shares some of the characteristic properties of the bottom water: high concentration of suspended particles, low concentrations of dissolved oxygen, chlorophyll-a, and phaeophytin. The particle size distributions in the intermediate nepheloid layer are different from those in the clear water above the nepheloid layer but similar to those in the bottom nepheloid layer. Two hypotheses for the generation of intermediate nepheloid layers, settling and horizontal advection, are examined, and the data support the latter hypothesis.