The evolution of etch pits on the AA1050 aluminum lithographic plate was studied. AC electrograining was carried out in a 35 degreesC, 0.16 M hydrochloric acid solution at a current density of 15 A(peak) dm(-2) and a frequency of 50 Hz. Scanning electron microscopy was used to characterize the surface morphology of electrograined aluminum. An epoxy replica technique was employed to reveal the internal structure of etch pits. Cross-sectional transmission electron microscopy was performed to reveal the detailed pit morphology and the microstructure of etch film. Together with the measurement of surface properties such as R-a, R-max, peak count and capacitance, a pit growth sequence is described. Three types of pits are observed during the first stage of electrograining: the fine pit, hemispherical pit and worm-like pit. The mergence of fine pits results in hemispherical pit. Lateral and unidirectional growth of hemispherical pits creates worm-like pit. A uniformly pitted surface is achieved when most of the fine pits merge into hemispherical pits. At this stage of electrograining, the worm-like pits disappear as they coalesce with hemispherical pits. TEM observations further illustrate that the hemispherical pits formed up to this stage are shallow in base. With further electrograining, new hemispherical pits form on the walls of existing hemispherical pits, leading to the formation of deeper pits.