Colloid concentration is an important parameter in models of colloid-facilitated transport. The purpose of the present study is to characterize colloid concentrations and colloid stability in natural groundwater from the Horonobe Underground Research Laboratory (URL) in Hokkaido, Japan. The particle sizes of colloids in groundwaters from the Horonobe URL range from several nanometres to c. 500 nm, with a mode particle size of c. 120 nm. Evaluation of colloid stability by DLVO theory suggests that larger colloids (i.e. >100 nm in diameter) would be more stable than smaller colloids in some groundwaters. The estimated colloid particle concentrations when considering the results of DLVO calculations ranged from 2.33 × 10⁶ to 1.12 × 10⁸ particles/ml, and mass concentrations were estimated to range from 45 to 1540 µg l⁻¹ for diameters greater than 100 nm. Colloids in Horonobe groundwaters appear to be less stable, with a moderate potential for transport, than colloids investigated in similar international studies. This reduced stability may be due to relatively higher ionic strengths and moderate dissolved organic concentrations in Horonobe groundwaters compared to their international counterparts.