Several pollen-specific genes from different species have been isolated and characterized at the molecular level, but the precise role of most of them is unknown. Mutant analysis represents a direct approach to uncovering gene function, but the paucity of available mutants affecting pollen development and/or function and the poor characterization of the known mutants have so far limited the exploitation of this approach. Here we present the cytological characterization ofgametophytic male sterile-1 (gaMS-1), a maize mutant that we identified in a program of transposon insertion mutagenesis for the production of mutations in gametophytically acting genes involved in microsporogenesis.gaMS-1 is expressed during or immediately after the first microspore division and leads to the production of immature, nonfunctional pollen grains. The mutation appears to affect the events leading to the developmental switch that follows the first microspore mitosis.