It is widely accepted that the distribution function of the masses of young star clusters is universal and can be purely interpreted as a probability density distribution function with a constant upper mass limit.As a result of this picture,the masses of the most massive objects would be exclusively determined by the size of the sample.Conversely,we show that the masses of the most massive young(≤ 10 Myr)star clusters in the flocculent galaxy M33 decrease with increasing galactocentric radius,in contradiction to a constant upper mass limit of the cluster mass function.Moreover,by comparing the radial distributions of gas surface densities and most massive cluster masses,we find that the maximum cluster mass,Mmax,scales with the molecular and total gas surface densities,and with the star formation rate density as well.The change of the maximum cluster mass there must be due to physical causes,i.e.,very massive star clusters may require special physical conditions,like high gas surface densities,in order to form.