Thein vivo chlorophylla fluorescence technique for phytoplankton depends on the effective absorption and fluorescence quantum yield of chlorophylla. The range of variation in the ratio ofin vivo fluorescence: extractable chlorophylla was found to be nearly 10-fold in natural Chesapeake Bay phytoplankton populations with significant areal and seasonal differences in addition to ambient light dependent fluorescence inhibition. Diel variations in chlorophylla fluorescence ratios were found to be species specific responses of some diatom and dinoflagellate populations. Observed changes in fluorescence ratios of several natural populations from day to night levels were as great as 8-fold when diatomsCerataulina bergonii andRhizosolenia sp. dominated the phytoplankton biomass. The diel response was changes in ambient light, the Kautsky Induction Effect, were associated with the degree of nutritional stress in natural samples and in unialgal cultures of phytoplankton. These observations may serve to explain some of the variations encountered in the application of thein vivo chlorophylla fluorescence technique and to provide a basis for an assessment of the physiological conditional of natural phytoplankton populations. Differences in cellularin vivo fluorescence ratios in vertical profiles of natural populations which might have been associated with variations in species composition variation with depth were eliminated by uniform dark treatment and suggest the observedin vivo chlorophylla fluorescence is in part related toin situ adaptation to the spectral quality of ambient light.