A process comprising continuously contacting with a catalyst, under polymerization conditions, ethylene or a mixture comprising ethylene and one or more alpha-olefins in a first fluidized bed reactor and a mixture comprising ethylene and one or more alpha-olefins in a second fluidized bed reactor, the first and second reactors being connected in series, said catalyst comprising:(A) a catalyst precursor comprising:(i) a vanadium compound, which is the reaction product of(a) VX.sub.3 wherein each X is independently chlorine, bromine, or iodine; and(b) an electron donor, which is a liquid, organic Lewis base in which VX.sub.3 is soluble;(ii) a modifier having the formula BX.sub.3 or AlR.sub.(3-a) wherein each X is as defined above; each R is independently alkyl having 1 to 14 carbon atoms; and a is 1 or 2; and(iii) a silica or alumina support for said vanadium compound and modifier;(B) a cocatalyst having the formula AlR.sub.3 wherein R is as defined above; and(C) a promoter having the formula R'.sub.b CX'.sub.(4-b) wherein R' is hydrogen or an unsubstituted or halo substituted alkyl having 1 to 6 carbon atoms; X' is a halogen; and b is 0, 1, or 2, wherein,in the first reactor,a homopolymer or copolymer is made having a relatively high melt index in the range of about 1 to about 100 grams per 10 minutes; a density of at least 0.950 gram per cubic centimeter; and a melt flow ratio in the range of about 50 to about 100; andin the second reactor,a copolymer is made having a relatively low melt index in the range of about 0.001 to about 0.01 gram per 10 minutes; a density of at least 0.925 gram per cubic centimeter; and a melt flow ratio in the range of about 50 to about 100,wherein, in the second reactor, the first reactor homopolymer or copolymer and the second reactor copolymer are blended in situ.