This article reviews the main factors affecting the in vivo fluorescence versus chlorophyll relationships of phytoplankton and presents a case study based on data from three oceanographic cruises carried out, at different times of the year, in the Catalan-Balearic Sea. In al1 three surveys, the in vivo fluorescence/chlorophyll ratio of the upper euphotic layer samples presented a diel variability with a minimum at or before noon time. The relationships between the spatio-temporal distribution of this variability and characteristics of photosynthesis versus irradiance curves obtained during each cruise are discussed.