We analysed data from three Bulgarian reservoirs, with trophic status from meso to eutrophic . Two kinds of relations (non-allometric and allometric) of chlorophyll with phytoplankton density, biovolume and surface area were investigated . The non-allometric relationships compare these phytoplankton variables with chlorophyll-a concentration, while the allometric ones include comparisons of average individual volume (AIV) of phytoplankters with chlorophyll-a content per individual (Chl .N -1 ), per unit biovolume (Chl .BV -1 ) and per unit surface area (Chl .SA-1 ) .Maximum values of the rank correlation of the three non-allometric relationships were found in the eutrophic reservoir. The highest allometric correlation coefficients were observed in the mesotrophic reservoir. Two nonallometric relationships, Chl with N and Chl ith BV calculated on unified data from all three reservoirs showed lower significance and a non-linear character . Chl-SA relationship was always statistically significant and varied from linear to slightly non-linear when calculated on weighed values . Two allometric relationships, AIV with Chl .BV -1 and AIV with Chl .N -1 seem to be linear. A third AIV and Chl .SA -1 was described by a polinomial of the second degree, indicating that the smallest and largest phytoplankton individuals have a similar chlorophyll content per unit surface area . At the highest trophy, the Chl .SA-1 seemed lowest .The frequently investigated relationship between chlorophyll-a and biovolume ranged from statistically not significant in the mesotrophic to highly significant in the eutrophic reservoir . This tendency was generalized by obtaining a statistically significant rank correlation between the levels of significance of chlorophyll-biovolume correlations and the corresponding chlorophyll-a concentrations . The non-linear character of the chlorophyllbiovolume relationship over a wide range of trophy was probably caused by changes in surface area-biovolume ratio .