A system comprising: a paraffin feed produced by the Fischer-Tropsch process is fractionable into at least three fractions: an intermediate fraction boiling between T1 and T2, T1 being in the range 120 200.degree. C. and T2 being more than 300.degree. C. and less than 410.degree. C., a light fraction boiling below it and a heavy fraction boiling above it; at least a portion of the intermediate fraction is hydrotreated then at least a portion thereof is passed over an amorphous hydroisomerisation/hydrocracking catalyst; the heavy fraction is passed over an amorphous hydroisomerisation/hydrocracking catalyst with a conversion of 370.degree. C.+ products into 370.degree. C.- products of more than 80% by weight; the hydrocracked/hydroisomerised fractions are distilled to obtain middle distillates (kerosine, gas oil).