Network-firm, hollow corporation, fragmented firm: all these terms suggest the recent emergence of new types of firm. This transformation appears on site, in the modes of localization of business activities. In this article the author examines the reasons why employees work off the firm's premises (industrial firms of 50 employees or more). Their position in the firm clearly plays a key part. Generally, we witness a slackening of control over work times and new modalities of collective work. While computer use in the working environment reinforces confinement to the firm's premises, the telephone is characteristic of mobile workers' equipment.