The majority of work on protection in single-language mobile code environments focuses on information security issues and depends on the language environment for solutions to the problems of resource management and process isolation. We believe that what is needed in these environments are not ad-hoc or incremental changes but a coherent approach to security, failure isolation, and resource management. Protection, separation, and control of the resources used by mutually untrusting components, applets, applications, or agents are exactly the same problems faced by multi-user operating systems. We believe that real solutions will come only if an OS model is uniformly applied to these environments. We present Alta, our prototype Java-based system patterned on Fluke, a highly structured, hardware-based OS, and report on its features appropriate to mobile code. 1 Operating System Model Required In the last EuropeanSIGOPS Workshop, our paper [17] arguedthat the local operatingsystem is an ...