The results of two Daedalus Airborne Thematic Mapper surveys of phytoplankton chlorophyll in six large lakes and ten small tarns are described. The water bodies surveyed covered a range of trophic states, but were all 'Case 1' waters whose optical properties were primarily determined by the concentration of phytoplankton in suspension. The surveys were supported by in situ measurements of phytoplankton chlorophyll and radiometric measurements of the upwelling radiance at selected sites. Statistical analyses showed that radiance measurements in ATM Band 3 (520-600 nm) provided the best estimates of phytoplankton chlorophyll in the more productive waters (correlation coefficient r = 0.97). In waters where the measured concentration of chlorophyll was less than 10 mu g l -1, the most effective pigment retrieval algorithm ( r = 0.91) was that based on the ratio of the radiances in ATM Band 3 and ATM Band 2 (450- 520nm). The two algorithms were then used to estimate the average concentration of phytoplankton in each water body and to produce maps of the spatial distribution of chlorophyll in the larger lakes. Chlorophyll concentrations could be predicted with an overall accuracy of +/- 3 mu g l -1 ( r = 0.98), and spatial distribution maps produced with contour intervals of 2-10 mu g l -1.