The aggregating states of octadecyltrimethylammonium bromide, C18TAB on mica surfaces were investigated by using AFM. The C18TAB physisorbed and/or chemisorbed on mica surface from aqueous solutions by dipping aggregates to various forms depending on the concentration of the solutions. The aggregates formed discontinuously on the surface from dilute solutions, while from concentrated solutions just below cmc, the aggregates covered the surface continuously. Most of the aggregates were easily desorbed with rinsing water and only remained ion-exchanged C18TA⁺ ions on the mica surface. On the mica surface immersed in high concentration solution, ion-exchanged by C18TA⁺ ions occurred almost on the whole surface. Thus the aggregates formed regularly in array.