Petunia embryos carrying the no apical meristem (nam) mutation fail to develop a shoot apical meristem. Occasional shoots on nam- seedlings bear flowers that develop ten instead of five primordia in the second whorl. Double mutants with the homeotic gene green petals show that nam acts independently of organ identify in whorl 2 and now also affects primordium number in whorl 3. The nam gene was isolated by transposon tagging. The encoded protein shares a conserved N-terminal domain with several other proteins of unknown function and thus represents a novel class of proteins. Strikingly, nam mRNA accumulates in cells at the boundaries of meristems and primordia. These data indicate a role for nam in determining positions of meristems and primordia.