The author marked and followed individual plants of Linanthus androsaceus (a California grassland annual) throughout the flowering season, recording individual flowering phenology, flower number, mortality, and seed production. Although data of first flowering was unrelated to number of flowers, plants first flowering during an intermediate interval had a greater probability of setting seed, and a stronger relationship between seed number and flower number, than plants first flowering early or late in the season. Flowering duration was correlated with flower number, with a positively skewed distribution that reflected the skewed size structure of the population. Although individual variation in first flowering date was related to reproductive success in L. androsaceus , the size dependence of flowering duration provides a mechanism whereby ecological factors can shape population flowering phenologies without evolutionary change.