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Serotiny, or delayed seed dispersal, is common in fluctuating environments because it hedges the risks of establishment. Mammillaria pectinifera (Cactaceae) facultatively expels fruits in the year they are produced or retains them to disperse the seed over several years. We tested whether M. pectinifera increased fruit expulsion as a response to increased rainfall. While no fruit expulsion was observed in 1997. a dry year, in the wetter 1998 around 20% of all fruits formed were expelled from the maternal plant. A greenhouse experiment showed thin high moisture results in the plants expelling all their fruits. Because in 1998 establishment was five times higher than in 1997 this response seems to be highly adaptive: Active fruit expulsion and consequent seed release increases the probability of establishment during pulses of high precipitation.