In two saline turbid mining lakes, the seasonal dynamics of autotrophic picoplankton (APP) were investigated. APP biomass accounted for 10–96% (annual average 48%) of the total autotrophic biomass. Eucaryotic picoplankton was dominant during spring and autumn (0.4–9 107 cells l-1). Picocyanobacteria appeared from June to September, reaching abundances of up to 1.3 × 108 cells l-1. The total phosphorus content varied in the range 7–46 µg P l-1 and chlorophyll a (chla) in the range 0.5–5.2 µg chla l-1. The euphotic zone (1%-light level) extended only over the upper part of the epilimnion during the 1997 growing season. Red light (620–720 nm) was available within the water body. Blue or green light (430–580 nm) was mostly absent because of the high contents of suspended particles in lakes. This may be a reason for the occurrence of only phycocyanin-rich picocyanobacteria in the lakes investigated. This study shows that the APP constitutes a significant part of the phytoplankton in these slightly saline (6.5–8%) turbid mining lakes. Because of their small cell size, higher uptake rates of inorganic nutrients and possibility of efficient utilization of low light intensities, they might have had an advantage for the development in the lakes investigated.