ABSTRACTThe use of male sterile lines is one of the ways to reduce the selfing rates and thus maximize heterosis in alfalfa (Medicago sativa L.). A spontaneous, stable male sterile mutant ms1 has been derived from M. sativa ‘Graze401’. In this paper, the microsporogenesis and microgametogenesis have been examined in ms1 and wild‐type plants under light and scanning electron microscopy. There were no significant differences of anther development between ms1 and the wild‐type plants until the end of the tetrad stage. The abortive development of anthers was initiated no later than the young microspore stage, as indicated by the appearance of large vacuoles in tapetal cells and the delayed tapetum degeneration leading to abortion of microsporogenesis. The shrunken shape and abnormal smooth surface of pollen walls in ms1 was observed, in which an incomplete exine formation on sterile microspores may indicate insufficient deposition of pollen kitt from the tapetal cells as well as microspores. Abnormalities of male meiosis in ms1 included lagged chromosomes, chromosome bridges, scattered chromosomes, micronuclei, and asynchronous chromosome segregations during meiosis II. We concluded that this new male sterile mutant is stable with normal stamen development but complete pollen abortion at a late stage of microsporogenesis.