The specific activities of 137Cs and 134Cs in the waters and sediments of Windermere North Basin (WNB) and Esthwaite Water (EW) in the English Lake District were determined over an 18-month period, immediately following the catastrophic accident at the Chernobyl nuclear reactor. Input of Chernobyl-derived Cs to the lake surfaces (2000 Bq of 137Cs m−2) occurred predominantly through direct atmospheric deposition over the period 3–20 May, 1986. The initial and highest specific activities (Co) of 137Cs were 80 Bq m−3 and 390 Bq m−3 in WNB and EW, respectively, reflecting the greater mean depth of WNB (25.1 m) compared with EW (6.4 m). Of the initial input, an estimated 37–41% and 32% was hydraulically flushed from WNB and EW, respectively, the remainder accumulating in the sediments. Retention half-life within the lake waters was 70 days in WNB and 15 days in EW. The temporal decline in caesium in surface waters could be modelled by assuming that either direct adsorption to the sediments or association with settling particles occurred in conjunction with hydraulic flushing. Model fits resulted in values of 0.10 ± 0.05 cm for the boundary layer thickness, and 105 litre kg−1 for the partition coefficients. Kd, in both lakes, indicating that transport by particles may be the dominant process. A small fraction (0.25%) of the caesium accumulating in the sediment appeared to be remobilized into the overlying waters when they became anoxic. This was the major source of radiocaesium in EW after April 1987 (ca. 1 year after the initial input).