The concentration of chlorophyll in laboratory grown cultures of marine phytoplankton and in oceanic samples has been determined both by measurement of fluorescence and by measurement of light absorption. The lower limit for detection of chlorophyll by fluorescence with the instrumentation described is about 0·01 μg chlorophyll a, which is about 5% that required for a spectro-photometric determination. Through choice of appropriate filters, the amount of fluorescence reflects either the chlorophyll a concentration or the sum of chlorophylls a and c. By measurement of fluorescence before and after acidification, the ratio of chlorophyll to phaeophytin can be readily determined. Dilute HCl is superior to oxalic acid for acidification of pigment extracts. As the fluorometric determination of chlorophyll and phaeophytin is fast, reliable and sensitive, it will be very useful in field studies of productivity.