During the Tulipmania from 1633 until 1637, the bloemisten used their preexisting professional networks to trade tulips with one another. This was no exception for the Mennonite bloemisten from Haarlem, such as Jacques de Clercq, Vincent Casteleyn, or Abraham van Meeckeren. This article will give an overview of these preexisting professional networks which they used. These networks were established mostly in the decades leading up to the Tulipmania, which is why the network analysis will cover these first decades up to and including the period of the Tulipmania. This article will particularly give an overview of the Mennonite textile industry, the book publishing family Casteleyn, and breweries in Haarlem. All these professional networks became even more interconnected through friendships and tactical marriages.