Effects of iridium doping on the differences in photographic behavior between cubic and octahedral AgBr grains have been investigated. The experimental AgBr grains were prepared without spectral or chemical sensitization. The iridium ions were incorporated into the major shell region of each grain (shell/core = 36/1) with different concentrations of iridium (0 ∼ 10⁻⁵ mol Ir/mol Ag). Regardless of the iridium doping amount, it was observed that latent images of cubic grains are almost all located at the surface, while in octahedral grains some latent images are also located inside the grains. These features are explained by relat-ing the latent image position to the potential difference across the space charge layer in octahedral and cubic grains. Undoped octahedral grains exhibited reduced effectiveness in latent image formation than undoped cubic grains. Introduction of iridium doping (with a concentration of 10⁻⁶ mol Ir/mol Ag) remarkably eliminates this reduced effectiveness observed for octahedral grains. In addition, iridium doping gives the octahedral grains a higher sensitivity increase during delayed latent image formation, compared to cubic grains. Such behavior is discussed in terms of a greater number of electrons being temporarily trapped by iridium centers.