In this paper, we prove that the most important concept of supervisory control of discrete-event systems, the controllability property, is undecidable for two deterministic context-free languages K and L. where L is prefix closed, even though K is a subset of L. If K is not a subset of L, the undecidability follows from the work by Sreenivas. However, the case where K is a subset of L does not follow from that work because it is decidable whether K and L are equivalent as shown by Senizergues. Thus, our result completes this study. The problem is also mentioned as open in the Ph.D. thesis by Griffin, who extended the supervisory control framework so that the specification language is modeled as a deterministic context-free language (compared to the classical approach where the specification is regular) and the plant language is regular. This approach is of interest because it brings an opportunity for more concise representations of the specification (as discussed, e.g., in the work by Geffert et al.) and, therefore, in some sense it treats the most interesting problem of current supervisory control theory, the state-space explosion problem. (C) 2012 Elsevier Ltd. All rights reserved.