A continuous vapor-phase fluidized-bed process for the preparation of ethylene homopolymers and copolymers having a density of from 0.87 to 0.97 g/cm3 in which ethylene or mixtures of ethylene and C3-C8 a-monoolefins are (co)polymerized in the presence of a supported chromium catalyst in the polymerization zone of a vapor-phase fluidized-bed reactor under pressures ranging from 1 to 100 bar and at temperatures ranging from 30null to 125null C. in the vapor phase in an agitated bed of bulk material comprising particulate polymer, the resultant heat of polymerization is removed by cooling the recirculated reactor gas and the resulting (co)polymer is removed from the vapor-phase fluidized-bed reactor, wherein, for the preparation of a (co)polymer of a specific density d, the (co)polymerization is carried out at a temperature which is in a range restricted by an upper envelope defined by equation I 1 T H = 171 + 6 null null null d null 0.84 - d null ( I ) and a lower envelope defined by equation II 2 T L = 173 + 7.3 null null null d null 0.837 - d null , ( II ) in which the variables have the following meanings: TH is the highest reaction temperature in null C.; TL is the lowest reaction temperature in null C.; dnull is the numerical value of the density d of the (co)polymer to be synthesized.