Genetic variation in flowering time was studied in four natural populations of Arabidopsis thaliana, using greenhouse experiments. Two populations from ruderal sites flowered early, two others from river dykes late. However, the late flowering plants flowered almost as early as the others after cold treatment of the seeds. The observed differences in flowering time have consequences for the potential life cycles of plants in the field. Plants from ruderal sites can be summer or winter annual, perhaps even have two generations per year. The strong response to cold treatment of plants from the dyke vegetations makes their life cycle strictly annual. Segregation analyses of the F2-s from crosses between early and late plants suggests that this variation in flowering time was genetic, with a dominant major gene for late flowering. Lateness in combination with vernalization responsiveness appears to be mostly due to a single gene in A. thaliana. It is hypothesized that early flowering ecotypes have evolved from late flowering ecotypes in different habitats by a loss mutation in the inhibitory mechanisms that delays flowering until after a period of low temperatures.