The development process of five kinds of silver iodobromide photographic emulsion grains having different iodide distributions was investigated with electron microscopic observations of the grains after partial development. The emulsions were not chemically digested intentionally. The exposure was made to the thinly coated emulsion layer. The developer was of a diluted surface developer. The results are summarised as follows; 1) Pure silver bromide emulsion grains (cubic surface).-Their edges and corners were the preferred sites for development centre formation. 2) Silver iodobromide emulsion grains with iodide-rich phase at the grain core (cubic and octahedral surface).-The development proceeded from one or two sites on their side face. The smaller grains were more rapidly developed than the larger grains, and the development centres were numerous on smaller grains. 3) Silver iodobromide emulsion grains in which iodide was most homogeneously distributed among five (cubic and octahedral surface).-There were found two kinds of grains. The one, with cubic surface, was developed in the similar fasion to pure silver bromide, and the other with octahedral surface, was little developed. 4) Silver iodobromide emulsion grains containing the silver iodide rich phase between the surface and core.-Development centres were formed on their side face (5-6 centres per one grain surface), quite different from that on pure bromide or iodobromide emulsions above mentioned. 5) Silver iodobromide grains with the iodide-rich phase on the surface.-Surface developer could not act on them (the development of them did not proceed). It appears that the iodide incorporation of different amount and/or distribution brings different amount and/or distribution of the development centres.