A Lagrangian drifter was deployed in a cold filament off northern California as part of the Coastal Transition Zone program. The drifter was equipped with an optical package (consisting of a spectroradiometer, a fluorometer, and a beam transmissometer) suspended at 8.5-m depth and a water sampler suspended at 16.3-m depth. The drifter was recovered after 8 days. Optical, chemical, and biological properties changed considerably as the drifter moved offshore in the cold filament. Concentrations of phytoplankton chlorophyll increased rapidly in the first 2 days, in parallel with the disappearance of nitrate and nitrite. After this initial period, chlorophyll decreased gradually over the next 6 days with prominent diurnal fluctuations present in the 3 days. Water transparency also showed similar long-term as well as diurnal fluctuations. The phytoplankton community became increasingly dominated by large centric diatoms throughout the deployment. Although total cell volume was higher towards the middle of the depolyment, this increase occurred without a parallel increase in chlorophyll. In addition, total particulate concentrations were highest nearshore. Although the drifter slippage was approximately 1 cm/s, the biological, chemical, and physical characteristics of the water were affected by both in situ changes and vertical motions of the water. These results are generally consistent with results from other upwelling studies. ¿ American Geophysical Union 1990