Variation in life-history traits such as emergence, survival, time of flowering, and fecundity were studied inDiplotaxis erucoides, a mediterranean winter annual weed, by analyzing cohorts that emerged in autumn, early spring, and spring. The response of the plants to the environment, as reflected by plant architecture and pattern of biomass allocation, was also studied. Seedlings that germinate in autumn produced from 3 to 10 times more seeds than those that germinated in spring. The main factor affecting the number of seeds produced appears to be the life-span. Reduction of the growing period led to a decrease in both number and length of modular units, which resulted in decreased numbers of leaves, flowers, and fruits of each module. In semelparousD.erucoidesplants, differences in the pattern of biomass allocation to reproduction are related to plant size. Our field data indicate that an increase of reproductive effort with size occurs in small individuals; however, a decrease occurs for vegetative biomass greater than 2 g and less than 5 g. Little variation in reproductive effort occurs when vegetative biomass is greater than 5 g. From a strategic point of view, size-dependent variation of reproductive effort inD.erucoidescan be interpreted as good tactics to favour a higher proportion of resources devoted to reproduction in small individuals, thus ensuring some offspring. However, structural, developmental, and physiological constraints lead to stabilization or even a decrease in reproductive effort above a certain threshold size, when production of offspring is already ensured.Key words: phenotypic plasticity, plant architecture, biomass allocation,Diplotaxis erucoides.