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Determinations of chlorophyll a and in vivo fluorescence of photosynthetic pigments were used to study vertical and horizontal distribution of phytoplankton in Lake Tanganyika (East Africa). Blue excited fluorescence (IVFb) was an approximate predictor of chlorophyll a at different depths and locations. Green excited fluorescence (IVFg), which reflects phycoerythrin in cyanobacteria, explained chlorophyll a variation equally well, and in combination with IVFb the degree of explanation was improved to 87% (n = 90). Particularly during the shallow stratification in March-May, the maxima of chlorophyll a, IVFb and IVFg were located within the thermocline. Such distribution may have resulted from the high penetration of UV light, often accentuated by very shallow daytime thermal stratification, leading to inhibition of phytoplankton near the surface. Because the decrease of chlorophyll a specific IVFb was less striking towards the surface, the decrease of IVFb was not caused by light inhibition only. In October-November, epilimnetic IVFb and chlorophyll a values seemed tobe consistently higher than in April-May and often showed remarkablepatchiness. The sometimes very dense phytoplankton blooms (Anabaena sp., Cyanobacteria) observed in the central and southern parts of thelake, suggest that local upwelling or mixing events may be importantfor the development of phytoplankton in Lake Tanganyika. [ABSTRACT FROM AUTHOR]