The utility is assessed of remotely-sensed chlorophyll data in biological oceanography. Oceanographic data, highly resolved in the vertical, are used as a basis for estimating the (weighted) proportion of the water-column chlorophyll chat is accessible to the remote sensor. Examples are calculated for the continental shelf off Nova Scotia, the Canadian Arctic and the coast of Peru. The analysis is extended to the estimation of phytoplankton production. Remotely-sensed data contain only a small (5 per cent of phytoplankton biomass and 11 per cent of the turnover), but surprisingly stable, fraction of the information for the water column. A modest ground-truthing programme is required to exploit these data to the best advantage.