Transport of cesium (Cs) was investigated in a saturated natural chalk fracture with an average equivalent hydraulic aperture of 129 mu m. The results show that Cs (inflow concentration of 0.22 mmol L(-1)) can be transported in its dissolved form and in association with montmorillonite Humic acid (HA) did not sorb Cs bur enhanced colloid-associated Cs transport by 12 5% in terms of breakthrough curve (BTC) recovery The BTCs clearly showed desorption of Cs from the fracture walls during the artificial rainwater (ARW)-injection period. Cesium transport associated with montmorillonite colloids was significant, with a maximum colloid-associated Cs C/C(0) (outflow-to-inflow concentration ratio) value of 16 6 +/- 1.1% during the tracer (colloids and LiBr)-injection period However, the relative contribution of colloid-associated Cs transport to total Cs transport was relatively low, amounting to 10 3 +/- 7% and 14 5 +/- 7% with montmorillonite (500 mg L(-1)) and the montmorillonite-HA (10 mg L(-1)) mixture, respectively Readsorption of Cs onto the colloids occurred immediately on switching from the tracer suspension to the background solution of AIM The significant colloid-associated Cs transport. the stripping effect of Cs from colloids. and the slow desorption of Cs from fracture walls reported in this study have important implications for risk assessments of Cs mobility in fractured carbonatic rocks