In pot and field experiments the entergence, budding, flowering and ripening of Arabidopsis thaliana were observed. The quantitative relationships between phenology and climatic factors were determined using mainly multiple regression methods. Duration of periods between sowing and emergence and between flowering and ripening may be treated as hyperbolic functions of mean temperature and therefore the usual degreeday method can be applied. In the period between emergence and flowering the relationships are more complex; some modifications of methods relating development to temperature were used. A short vernalization after emergence, thereafter higher temperatures at long days accelerate flowering.