The “Aryanization” of the Belgian economy ordered by the German occupants aimed at the liquidation of virtually all small and medium-sized businesses owned by Jews. Most of these business owners had immigrated to Belgium from eastern Europe. The article deals with the genesis of this immigrant economy, presenting new findings on Jewish immigration to Belgium and examining the structure of Jewish businesses, as well as their distribution in Antwerp and Brussels. Statistically processed mass data serve as the basis for this, though individual case histories are also presented. The businesses of Jewish immigrants were mostly small family businesses and of an artisanal or proto-industrial nature, specializing on a few branches of trade and unable to secure for their owners more than a precarious existence. The article, which is being published in two parts, concludes with quotations from petitions business owners addressed to the German military administration in an effort to ward off the elimination of their livelihoods.