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A phenological assessment of Tortula inermis (Brid.) Mont. populations in the Mojave Desert, Nevada, U.S.A., over a period of 4 years revealed that the sporophyte cohort initiated in early 1995 remained dormant until early 1998, by which time approximately 66% of the sporophytes had aborted. The viable sporophytes of this cohort were significantly shorter in length and had significantly less biomass than the previous cohort of sporophytes. In the intervening years 1996 and 1997, the sexual reproductive cycle was skipped altogether. The absence of sporophyte initiation in these two years was attributed to reduced winter-spring rainfall. The majority of abortive sporophytes aborted in the late embryonic and seta elongation phenophases. The 1997 and 1998 summers were characterized by unusually heavy rains. Following the heavy summer rain events of 1997, sporophyte abortion in the 1995 cohort increased from 9 to 43%, and abortive sporophytes in this cohort further increased to 66% following the summer rains of 1998. Abortive sporophyte biomass averaged 49 mug or 33% of the mean biomass of the viable sporophytes in the cohort at maturity. These data suggest that the summer rain events precipitated two waves of sporophyte abortion, possibly through heavy membrane damage associated with rapid drying and high temperatures while hydrated.