The histology and time course of hygromycin-resistant callus formation from bombarded, seed embryo scutellum derived embryogenic nodular units (ENUs) were investigated in four japonica rice cultivars (Taipei309, IDSA6, IR47686, IRAT361) exhibiting contrasting amenabilies in a microprojectile-mediated transformation system. ENUs from the above genotypes collected at a stage considered suitable for bombardment, exhibited contrasted organization in their peripheral regions. This organization was found to influence the hygromycin concentration requested to stop ENU growth, along with the amenability of transformed cells to evolve into isolated, clonal, transgenic resistant calli, from the rest of the ENU. An average of 2.5, 4.3, and 3.2 hygromycin resistant calli were produced per responding ENU in Taipei 309, IDSA6 and IR47686, respectively. We obtained histological and molecular evidence that the hygromycin resistant calli arising from a single bombarded ENU may have been formed through independent transformation events