A process for selectively producing C.sub.2 to C.sub.4 olefins from feedstock such as a gas oil or resid. The feedstock is reacted in a first stage comprising a fluid catalytic cracking unit wherein it is converted in the presence of a mixture of conventional large pore zeolitic catalyst and a medium pore zeolitic catalyst to reaction products, including a naphtha boiling range stream. The naphtha boiling range stream is introduced into a second stage where it is contacted with a catalyst containing from about 10 to about 50 wt. % of a crystalline zeolite having an average pore diameter less than about 0.7 nanometers at reaction conditions which include temperatures ranging from about 500 to about 650.degree. C. and a hydrocarbon partial pressure from about 10 to about 40 psia (about 70 to about 280 kPa).