The role of transparency on the diurnal distribution of plant-attached cladocerans was studied in two similar-sized lakes with contrasting water colour. The diurnal attachment behaviour of Sida crystallina (O.F. Müller, 1776) was more pronounced in the less humic lake where the animals remained fixed on plants, indicating that staying attached was a more profitable option. In the highly humic lake, the pattern was opposite, and regardless of time the highest density observed was in the free-swimming individuals, with only few animals attached to the floating-leaves for refuge, indicating that low transparency provided sufficient protection against predation. The attached S. crystallina were larger compared to free-swimming individuals in the more transparent lake, suggesting greater vulnerability of large-sized individuals to predation. The results indicate that increasing concentrations of humic substances affecting the light environment may alter the diurnal behaviour and habitat use of plant-attached zooplankton.