Summary: The uses-hierarchy of a Smalltalk program’s packages is not easily visible to its maintainer. This sets the stage for a common error ‒ extending a low-level package in a way that makes it depend on a higher level package. Such a mistake introduces a cyclic dependency, which prevents the low-level package, and all others in the cycle, from being reused independently. This paper describes a tool called MudPie that uses well-known techniques to visualize the dependency structure as it is reflected in the code. We apply these techniques to Smalltalk and show how SUnit tests can detect the cycles as they occur. This can help programmers learn a system’s package hierarchy and avoid breaking it.