A process for conversion of a hydrocarbon feedstock comprising a relatively heavy main feedstock with a boiling point above approximately 350.degree. C., and a relatively light secondary feedstock with a boiling point below approximately 320.degree. C., wherein, the main feedstock, representing at least 50 wt. % of the hydrocarbon feedstock, is cracked in a fluidized-bed reactor in the presence of a cracking catalyst, the secondary feedstock is cracked in a fluidized bed with the same cracking catalyst, separately or mixed with the main feedstock, said secondary feedstock comprising oligomers with at least 8 carbon atoms of light olefins with 4 and/or 5 carbon atoms.